help: provide a more helpful message when no keyword are matched
Before this changeset, when no topic were matching the provided keyword ( in
command such as `hg help --keyword babar`) the output was empty. This is
confusing and unhelpful for new users seeking assistance in the help.
We now display:
$ hg help --keyword babar
abort: no matches
(try "hg help" for a list of topics)
This send users in a new direction where they may find what they are looking for.