help: show section that couldn't be found

For better error indication.

mercurial/commands.py

@@ -3916,6 +3916,7 b' def help_(ui, name=None, **opts):'
     if ui.verbose:
         keep.append('verbose')
 
+    fullname = name
     section = None
     subtopic = None
     if name and '.' in name:
@@ -3938,7 +3939,7 b' def help_(ui, name=None, **opts):'
     # to look for, or we could have simply failed to found "foo.bar"
     # because bar isn't a section of foo
     if section and not (formatted and name):
-        raise error.Abort(_("help section not found"))
+        raise error.Abort(_("help section not found: %s") % fullname)
 
     if 'verbose' in pruned:
         keep.append('omitted')

tests/test-help.t

@@ -1547,11 +1547,11 b' Test section lookup'
          "default:pushurl" should be used instead.
   
   $ hg help glossary.mcguffin
-  abort: help section not found
+  abort: help section not found: glossary.mcguffin
   [255]
 
   $ hg help glossary.mc.guffin
-  abort: help section not found
+  abort: help section not found: glossary.mc.guffin
   [255]
 
   $ hg help template.files