Fix sub-menu capability for plugins...

1. xrange(x, y) goes from x to y-1. If len(MENU) is 1 this is never
called. So add 1 to get correct range.

2. Exception handling is typically slower and code is less clear than if
we just test for the key existing.
Minor style improvement
Merge pull request #269 from Sophist-UK/sophist_fix_plugin_MENU_capability

Fix sub-menu capability for plugins...