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.

mercurial/extensions.py

@@ -69,7 +69,9 b' def load(ui, name, path):'
             return mod
         try:
             mod = importh("hgext.%s" % name)
-        except ImportError:
+        except ImportError, err:
+            ui.debug('could not import hgext.%s (%s): trying %s\n'
+                     % (name, err, name))
             mod = importh(name)
     _extensions[shortname] = mod
     _order.append(shortname)