`gui/mod-manager` does not validate return from `get_mod_info_metadata` before using

at line 527:
`local steam_id = scriptmanager.get_mod_info_metadata(mod.path, 'STEAM_FILE_ID').STEAM_FILE_ID`

this results in a Lua exception if `get_mod_info_metadata` returns `nil`, which can happen if the mod has a missing or defective `info.txt` file

code should check for a `nil` return from `get_mod_info_metadata` and degrade appropriately