Make the Processing thread survive errors from messages

[grundleborg]

The processing thread needs to handle exceptions so it can survive the occurrence of an error when processing a message. The listener interface should probably also gain a "on_error" method that allows the library user to get notified when decoding a message fails for some reason.

[grundleborg]

Probably makes more sense to notify the Monitor interface (#21), not the Listener interface.

[grundleborg]

I think this should now be OK. Need some buggy messages to come through to be sure though.