Implement checks for raising messages with exceptions

[N-Parsons]

Resolves #1080 by adding checks that all exceptions are raised with messages. The aim is to encourage learners to raise meaningful messages, but testing for meaningfulness isn't feasible, so this instead just tests that a message of non-zero length is passed when raising an exception.

TODO:

• Update any failing examples
• Add section to the README insert to explain the need for meaningful exception messages
• Regenerate READMEs to include the new insert

[N-Parsons]

I've held back the change to the proverb exercise description due to significant changes and put these in a separate PR (#1114) with the associated changes to the test-suite. This means that there will be a merge conflict, but I should be able to quickly resolve this once one is merged. (EDIT: Conflict resolved)

[cmccandless]

If you can confirm that most/all changes to README files (besides the insertion of the new Exception Messages section) are simply reflecting the latest versions of the relevant problem descriptions, then LGTM.

[N-Parsons]

@cmccandless, all changes reflect changes to the upstream problem descriptions. I had a look through and nothing significant appeared to be changing (except in proverb, hence #1114), so the changes shouldn't cause issues.

[stale]

This issue has been automatically marked as abandoned because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.