CoverageException is fine as a base class, but not good to use for
raising (and catching sometimes).  Introduce specialized exceptions that
allow third-party tools to integrate better.

The comprehension would only completely finish (->exit) if the assert
was going to fail.  So we expect it to not finish.

The old way, extra warnings that we don't care about could creep in.  For some
reason, disabling PyContracts causes "imp" DeprecationWarnings to appear in the
list.

Rather than assert there's only one warning, assert there's only one from us.

This makes coverage warnings visible when running test suites under
pytest.  But it also means some uninteresting warnings would show up in
our own test suite, so we had to catch or suppress those.