Make nested-trans-block exceptions nicer

[akx]

• Refs: Hide extensive traceback from the user and show only relevant information python-babel/babel#1049

Checklist:

• 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.
• 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.
• Run pre-commit hooks and fix any issues.
  • There is an unrelated flake8 failure.
• 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.