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.

mercurial/help.py

@@ -481,6 +481,10 b' def help_(ui, name, unknowncmd=False, fu'
                 rst.append('%s:\n\n' % title)
                 rst.extend(minirst.maketable(sorted(matches[t]), 1))
                 rst.append('\n')
+        if not rst:
+            msg = _('no matches')
+            hint = _('try "hg help" for a list of topics')
+            raise util.Abort(msg, hint=hint)
     elif name and name != 'shortlist':
         i = None
         if unknowncmd:

tests/test-help.t

@@ -988,6 +988,13 b' Test keyword search help'
   
    qclone clone main and patch repository at same time
 
+Test unfound keyword
+
+  $ hg help --keyword nonexistingwordthatwillneverexisteverever
+  abort: no matches
+  (try "hg help" for a list of topics)
+  [255]
+
 Test omit indicating for help
 
   $ cat > addverboseitems.py <<EOF