Improve skipmemymissionyo in several ways, based on RedMageJoe's recent experience:
1. Allow spaces to be accepted in the mission prompt
2. If the requested mission can't be found, just abort rather than restarting the entire campaign
3. Clarify that the prompt needs a filename and add a message if the mission can't be found
4. Improve handling of the .fs2 extension
5. Fix a bounds check in `popup_input`
Goober5000
8613ccde76