Be more specific in metadata parsing error messages

[Ortham]

libloot used to provide a single function for loading the prelude, masterlist and userlist, so LOOT would display the same error message if any of them failed, but since libloot v0.26.0 it provides separate functions for them, so it's possible to display a different message if the masterlist and prelude failed, vs. just the userlist. It might also be worth falling back to just parsing the masterlist to see if the issue is with it or the prelude if loading them both fails.

The relevant code is https://github.com/loot/loot/blob/master/src/gui/state/game/game.cpp#L913-L968.