help: don't crash in keyword search if an extension fails to provide docs
Not all external extensions provide docs; if you use such an extension, you
will experience a crash if you use "hg help --keyword <word>", and <word>
happens to match the extension name.
$ 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