tests/sitecustomize.py

extensions: print some debug info on import failure

This is handy if hgext.foo exists but has a bogus "import blah":
previously we just discarded the "No module named blah" error. Now at
least you can see it with --debug. Not perfect, but better than
nothing.

try:
    import coverage
    getattr(coverage, 'process_startup', lambda: None)()
except ImportError:
    pass