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.

Addresses PICARD-580 http://tickets.musicbrainz.org/browse/PICARD-580.