Failure modes for missing Pygments are not good

With the Qt backend, if you don't have Pygments installed and try to use the `CodeEditor` in TraitsUI you get a `TextEditor`. With Pyface it's not quite so bad as you get an `ImportError` which, if you have access to the source code, will lead you to the problem.

Furthermore when running from the command-line in the most obvious way you see _no_ indication of what the problem is because the missing import message is sent out as debug-level and so is obscured.

It will be almost impossible for new users to work out what is going wrong.

I am leaning towards we should just fail loudly when the import of Pygments fails.