(#7282) action without when_invoked should fail...

We used to let actions be declared without the when_invoked block, which was
usually a sign of either someone writing their method code direct in action
declaration, or someone forgetting to add their code at all.

This was just let silently by: the error only showed up when you finally tried
to invoke the action, and a NoMethod error was raised by the face.

...except for our own testing. We took advantage of this a whole pile of
times in there; fixing the original UI issue means fixing all those too.

Reviewed-By: Nick Lewis <nick@puppetlabs.com>