Use R2DBC exceptions to communicate failures

[mp911de]

When operations fail with an error, the driver should use the R2dbcException hierarchy to signal failures.

Right now, plain Exception is used as error, see:

xception in thread "main" reactor.core.Exceptions$ReactiveException: java.lang.Exception: Database testdb1 not found.
	at reactor.core.Exceptions.propagate(Exceptions.java:408)
	at reactor.core.publisher.BlockingSingleSubscriber.blockingGet(BlockingSingleSubscriber.java:100)
	at reactor.core.publisher.Mono.block(Mono.java:1712)
	at com.example.db2.Db2Application.main(Db2Application.java:27)
	Suppressed: java.lang.Exception: #block terminated with an error
		at reactor.core.publisher.BlockingSingleSubscriber.blockingGet(BlockingSingleSubscriber.java:102)
		... 2 more
Caused by: java.lang.Exception: Database testdb1 not found.
	at com.ibm.db2.r2dbc.b.q.lambda$send$0(q.java:120)
	at reactor.core.publisher.FluxHandle$HandleSubscriber.onNext(FluxHandle.java:113)
	at reactor.core.publisher.MonoFlatMapMany$FlatMapManyInner.onNext(MonoFlatMapMany.java:250)
	at reactor.core.publisher.EmitterProcessor.drain(EmitterProcessor.java:537)
	at reactor.core.publisher.EmitterProcessor.subscribe(EmitterProcessor.java:253)
	at reactor.core.publisher.Flux.subscribe(Flux.java:8773)
	at reactor.core.publisher.MonoFlatMapMany$FlatMapManyMain.onNext(MonoFlatMapMany.java:195)
	at reactor.core.publisher.MonoCreate$DefaultMonoSink.success(MonoCreate.java:172)
	at com.ibm.db2.r2dbc.b.d.lambda$null$1(d.java:115)
	at reactor.core.publisher.FluxPeek$PeekSubscriber.onNext(FluxPeek.java:185)
	at reactor.core.publisher.FluxHandle$HandleSubscriber.onNext(FluxHandle.java:129)
	at reactor.core.publisher.FluxPeekFuseable$PeekConditionalSubscriber.onNext(FluxPeekFuseable.java:854)
	at reactor.core.publisher.FluxMap$MapConditionalSubscriber.onNext(FluxMap.java:224)
	at reactor.netty.channel.FluxReceive.onInboundNext(FluxReceive.java:377)
	at reactor.netty.channel.ChannelOperations.onInboundNext(ChannelOperations.java:411)
	at reactor.netty.channel.ChannelOperationsHandler.channelRead(ChannelOperationsHandler.java:113)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:444)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420)
	at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:412)
	at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1410)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:440)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420)
	at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:919)
	at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:166)
	at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:788)
	at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:724)
	at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:650)
	at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:562)
	at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:997)
	at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
	at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
	at java.base/java.lang.Thread.run(Thread.java:833)

or

Caused by: java.lang.Exception: Authentication failed for user: foo using security mechanism: USERID_PASSWORD.
	at com.ibm.db2.r2dbc.b.g.lambda$send$0(g.java:142)
	at reactor.core.publisher.FluxHandle$HandleSubscriber.onNext(FluxHandle.java:113)
	at reactor.core.publisher.MonoFlatMapMany$FlatMapManyInner.onNext(MonoFlatMapMany.java:250)
	at reactor.core.publisher.EmitterProcessor.drain(EmitterProcessor.java:537)
	at reactor.core.publisher.EmitterProcessor.subscribe(EmitterProcessor.java:253)
	at reactor.core.publisher.Flux.subscribe(Flux.java:8773)
	at reactor.core.publisher.MonoFlatMapMany$FlatMapManyMain.onNext(MonoFlatMapMany.java:195)
	at reactor.core.publisher.MonoCreate$DefaultMonoSink.success(MonoCreate.java:172)
	at com.ibm.db2.r2dbc.b.d.lambda$null$1(d.java:115)
	at reactor.core.publisher.FluxPeek$PeekSubscriber.onNext(FluxPeek.java:185)
	at reactor.core.publisher.FluxHandle$HandleSubscriber.onNext(FluxHandle.java:129)
	at reactor.core.publisher.FluxPeekFuseable$PeekConditionalSubscriber.onNext(FluxPeekFuseable.java:854)
	at reactor.core.publisher.FluxMap$MapConditionalSubscriber.onNext(FluxMap.java:224)
	at reactor.netty.channel.FluxReceive.onInboundNext(FluxReceive.java:377)
	at reactor.netty.channel.ChannelOperations.onInboundNext(ChannelOperations.java:411)
	at reactor.netty.channel.ChannelOperationsHandler.channelRead(ChannelOperationsHandler.java:113)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:444)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420)
	at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:412)
	at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1410)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:440)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420)
	at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:919)
	at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:166)
	at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:788)
	at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:724)
	at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:650)
	at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:562)
	at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:997)
	at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
	at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
	at java.base/java.lang.Thread.run(Thread.java:833)