netrc will now raise a more predictable exception when $HOME is not set

(as it is often not on Windows). The code was always designed so that it
would raise an IOError if there was no .netrc. But if there was no $HOME
it would return a KeyError which would be somewhat unexpected for code
that didn't know the algorithm it used to find .netrc. The particular
code that triggered this problem for me was ftpmirror.py which handled
the IOError gracefully, but not the KeyError.