Raise an error if cibuildwheel generates a pure/universal Python wheel

[joerick]

There's no reason to use cibuildwheel to build a universal/pure wheel, and in some cases (e.g. #254), the generation of a non-platform wheel means that compilation failed.

I'd expect an error message like:

cibuildwheel: Build failed because a pure Python wheel was generated.

If you intend to build a pure-Python wheel, you don't need cibuildwheel - use
`pip wheel -w DEST_DIR .` instead.

If you expected a platform wheel, check your project configuration, or run
cibuildwheel with CIBW_BUILD_VERBOSITY=1 to view build logs.