If an exception occurs while decoding a message, there is really
nothing the server can do to recover.  If the message was a
synchronous call, the client will wait for ever for the reply.  The
server can't send the reply, because it couldn't unpickle the message
id.  Instead of trying to recover, just let the exception propogate up
to asyncore where the connection will be closed.

As a result, eliminate DecodingError and special case in
handle_error() that handled flags == None.