Improve error handling and associated nullability annotations

[chrisballinger]

There are a number of methods (and inits) that silently fail and return nil without having an error parameter, and in conflict with the public nullability annotations.

[chrisballinger]

This assert should be an NSError:

SignalProtocol-ObjC/Classes/SignalSessionBuilder.m

Line 49 in bf612f5

NSAssert(result >= 0, @"couldn't process prekey bundle");