Ensure we don't end up with invalid exceptions again

We've recently had a few (three?) exceptions where the messages
didn't work because there was a typo in the exception variable
name. This change adds a very simple unit test which ensures that
we don't end up in that state again.

Change-Id: I00423b4ce82428959e7591d5c5c47c1f25ec3029
Merge "Ensure we don't end up with invalid exceptions again"