[Import-SIG] improving ImportError

[Eric Snow]

Armin Ronacher has an interesting blog post about some of the
challenges of using Python's imports:

http://lucumr.pocoo.org/2011/9/21/python-import-blackbox/

In particular, his main point about "not found" vs. "broken" vs.
"problem with inner import" got me thinking.  Would it be a bad idea
to split ImportError into more specific exceptions?  I imagine that
cataloguing the different causes of ImportError and how someone might
react to those differently wouldn't be too hard...

-eric