Assert on reversal errors

If there are no solutions found at this point, a bug is present causing unsymetric searches in clockwise vs counterclockwise. If no paths exists, this assert will never be reached anyway after the invalid_idx check

--Do not merge as such a bug is present --
Add comment on reversal assert
Merge pull request #430 from bliksemlabs/reversal_trainingwheels

Assert on reversal errors