#7211: more helpful error messages in various cases.

We were emitting a bunch of unhelpful failure messages, surrounding invalid
actions and especially default actions interacting with the command-line.

This cleans those up, to give a helpful, informative, and correct message in
all cases. Notably, we no longer report that there is no "default" action
when you specify an unknown action on a face.

This change revealed some other weaknesses in our unit tests, now correctly,
that result in slightly more robust code.