No longer throw if copying of single files fails during data migration.
Aiming to fix https://github.com/ppy/osu/runs/7601653833?check_suite_focus=true, which could also affect end users. I've left a limit before an exception is still thrown, to handle cases like the user running out of disk space (where we probably *do* want to bail, so they can continue to use their still-intact original storage location).
If this isn't seen as a good direction, an alternative will be to make the migration code aware of the structure of the temporary files created by `Storage` (but doesn't guarantee this will cover all cases of such temporary files – there could for isntance be metadata files created by the host operating system).
Another option would be to mark those temporary files as hidden and skip any hidden files during iteration.
