Remove unused imports
Throw RsyncProtocolException if Sender sends an invalid formatted Message to Receiver

Re-Throws TextConversionException as RsyncProtocolException if
Receiver cannot decode the Text message.

Also performs some trivial code cleanup when handling messages.
Fix issue #7: MessageHeader ERROR_XFER

Fixes https://github.com/perlundq/yajsync/issues/7

This adds support for handling of incoming messages to Sender.

TODO: introduces some code duplication (see Receiver).