Message 228587 - Python tracker

Committed the message changes to 3.5 only, since it will probably cause tests to fail in various projects, despite messages not being a formal part of the python API.  

Per IRC conversation with Ezio and Antoine, I posted a note to python-dev to let people know we now have a consistent terminology in the docs and error messages, and to provide a last opportunity for objections (it is easy enough to back the patch out if there is an outcry, but I don't expect one).