Don't attempt to auto-update mods if the previous attempt failed

:information_source: This is a more visible issue currently because Lönn doesn't allow mod zips to be edited while open.

When mods can't be auto-updated for any reason, restarting the game will again try to update them. This can cause an endless boot-loop when the issue affecting auto-update is persistent.

## Proposed Fix:
When an auto-update fails, create a file (e.g. `lastupdatefailed.txt`) that will be checked for before attempting to auto-update mods. The file would be deleted after any successful launch of the game, meaning every other launch would be successful.