Fix deprecation messages.

[anntzer]

1. in generate_deprecation message, pass addendum to the format dict
   (otherwise formatting %(addendum)s will throw).
2. in warn_deprecated in the rcParams, key is the name of the
   deprecated object, not the whole deprecation message.

release critical per point 1).

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is PEP 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[dstansby]

@anntzer can you reboot the circle 36 job? It's on your account so I can't do it myself.

[jklymak]

Circle is just having issues - I think we can go ahead and merge non-documentation PRs if every thing else passes...