mllp errback should return the failure to prevent callbacks continuing to be called after the error.

"Note: If an errback doesn’t return anything, then it effectively returns None , meaning that callbacks will continue to be executed after this errback."
https://twistedmatrix.com/documents/current/core/howto/defer.html#errbacks
Delegate error handling to the MessageContainer.
Bump version and add changelog for 0.2.1.