Doesn't work with hands that have less than 5 fingers (rigify)

[BradFitz66]

As far as I can tell, this will always error (complaining about missing finger bones) with rigify when trying clicking AutoGrip setup if I have a character with 4 or less fingers (and thus have had to delete the extra finger bones in the metarig). It would be great if this could be updated to support an arbitrary amount of fingers.