[m-rev.] for post-commit review: add color to purity diagnostics (and rewrite a few)

[Zoltan Somogyi]

For review by anyone. I would like feedback on the new wording
of many of the diagnostics.

Also, would anyone object if I renamed all the .exp files in tests/warnings
to end in .err_exp? I don't like having to context switch between the two
naming conventions all the time. Does anyone even remember the
original reason for the difference? I don't.

Zoltan.
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