Log exception on writing to and closing WritableByteChannel as warning

Currently, when an exception occurs while writing a representation to the byte channel, e.g. in a custom Jackson converter, the exception is swallowed and only logged at level 'fine'. The request completes successfully (with a 200 or 204), so it appear as if nothing went wrong.

Logging it at level 'warning' will at least make it visible in the console (with default settings). However, a better solution would be to let the exception propagate up the call stack, making it impossible to miss.