Make nested-trans-block exceptions nicer

[akx]

• Refs: https://github.com/python-babel/babel/issues/1049

Checklist:

• [x] Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• [ ] Add or update relevant docs, in the docs folder and in code.
  • Not relevant IMO.
• [x] Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• [ ] Add .. versionchanged:: entries in any relevant code docs.
  • Not relevant IMO.
• [x] Run pre-commit hooks and fix any issues.
  • There is an unrelated flake8 failure.
• [x] Run pytest and tox, no tests failed.

[davidism]

Nice idea, thanks for making this error message clearer! I'm not sure when this will get merged in, but it's on the list.