ActionFinish called after Action completion

The worker now calls ActionFinish on state to call home with the results of an Action.

The unit will not be set to an error state unless the API call to ActionFinish fails.  Any other errors generated by the Action will be encapsulated in the results message, which is sent back to state with the final state of the Action (complete or fail.)
It's not that it's rubbish per se.  It's simply a verbose error message indicating that there's a missing file, which gets encapsulated by the `missingHookError` type.  Perhaps annotating the error and returning _that_ would be the right thing for the user, but it seems more useful simply to indicate that the Action wasn't implemented.  However, that's not really the meaningful content from the perspective of what the error itself contains.  Willing to hear a pushback on this, of course.