Harden config file loading

[thashepherd]

Hardening this area a bit since I managed to crash the loader while testing AuroraDashboard with a copy/pasted mod.ini.

Bugs:

• Loader no longer crashes when a mod.ini points to a bad or nonexistent config file
• Loader no longer crashes if a mirror doesn't work / you're not connected to the internet
• Modals for both of the above with actionable information
• A 'mod download failed' modal to give the user some feedback when that happens (previously, it looked like something was happening)

Not pictured: AuroraLoader hard crashing Pictured: new helpful error message