Error message is misleading for deleting subtrip

[Haotian199]

If there are currently no subtrips in the list and user tries to delete a subtrip, the error message returned is index out of bounds. It would be better to use a more fitting error message name like there are no subtrips in the list.

[nus-pe-bot]

Your response not required for this bug as the team has accepted the bug as it is.