lobo path to return custom code

[PietroPasotti]

Adds a --retry flag so that jhack charm lobotomy can have the dispatch path exit nonzero. This is useful if you want juju to be aware that the charm isn't processing events. Usage:

• jhack lobotomy foo/0
• (do things), charm goes to error state
• jhack lobotomy foo/0 --undo
• juju resolve foo/0
• charm receives all queued events