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.
$ echo 'raise Exception("bit bucket overflow")' > badext.py
$ abspath=`pwd`/badext.py
$ echo '[extensions]' >> $HGRCPATH
$ echo "gpg =" >> $HGRCPATH
$ echo "hgext.gpg =" >> $HGRCPATH
$ echo "badext = $abspath" >> $HGRCPATH
$ echo "badext2 =" >> $HGRCPATH
$ hg -q help help
*** failed to import extension badext from $TESTTMP/badext.py: bit bucket overflow
*** failed to import extension badext2: No module named badext2
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