This ends up with a bit of an undefined behaviour, but it's already a
bit of an edge case (files missing in the `files` folder that are
references in the database).
First and foremost, let's stop the exception. If we allow it to throw,
it's impossible to exit the skin editor in this state.
Closes https://github.com/ppy/osu/issues/36135.
This fell out while investigating why the issue with online IDs
mismatching in the `.osu` could be worked around by importing the map
three times in total when starting from it not being available locally.
Here follows an explanation of why that "helped".
Import 1:
- The beatmap set is imported normally.
- Online metadata population sees the online ID mismatch and resets it
on the problematic beatmap.
Import 2:
- The existing beatmap set is found, but deemed not reusable
because of the single beatmap having its ID reset to -1.
- The existing beatmap set is marked deleted, and all the IDs of
its beatmaps are reset to -1.
- The beatmap set is reimported afresh.
- Online metadata population still sees the online ID mismatch
and resets it on the problematic beatmap.
Note that at this point the first import *is still physically present
in the database* but marked deleted.
Import 3:
- When trying to find the existing beatmap set to see if it can be
reused, *the one pending deletion and with its IDs reset -
- the remnant from import 1 - is returned*.
- Because of this, `validateOnlineIds()` resets online IDs
*on the model representing the current reimport*.
- The beatmap set is reimported yet again.
- With the online ID reset, the online metadata population check for
online ID mismatch does not run because *the IDs were reset to -1*
earlier.
Preferring undeleted models when picking the model instance for reuse
prevents this scenario.