Gracefully fail if something is already running on port 8005

[andrew]

Current output if you try to start forest server whilst something is already on 8005:

$ forest server
Connected to IPFS
Watching for new upstream releases for cached packages
node:events:356
      throw er; // Unhandled 'error' event
      ^

Error: listen EADDRINUSE: address already in use :::8005
    at Server.setupListenHandle [as _listen2] (node:net:1312:16)
    at listenInCluster (node:net:1360:12)
    at Server.listen (node:net:1447:7)
    at /Users/andrewnesbitt/code/forest/lib/commands/server.js:11:12
    at processTicksAndRejections (node:internal/process/task_queues:94:5)
Emitted 'error' event on Server instance at:
    at emitErrorNT (node:net:1339:8)
    at processTicksAndRejections (node:internal/process/task_queues:81:21) {
  code: 'EADDRINUSE',
  errno: -48,
  syscall: 'listen',
  address: '::',
  port: 8005
}

We can catch that error and show a nice message.

Related to #24 so users can configure a different port to use.