SanityCheck: allow RETURNTOHOME if pathfollower running

Only PositionHold was covered as a valid flight mode position
when the PathFollower is running.  RTH should also be allowable
in this configuration.
Sanitycheck: Error is "PathPlanner" when trying to use RTH or PH

This better represents that the error in both cases is the lack of
a PathPlanner module.
Merge pull request #563 from peabody124/rth_sanitycheck

SanityCheck: allow RETURNTOHOME if pathfollower running