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 "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state