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.
Sometimes I am not sure where my osu! is reading files from. This should
help somewhat.
```csharp
/Users/dean/Projects/osu/osu.Desktop/bin/Debug/net6.0/osu!
[runtime] 2022-07-13 07:22:03 [verbose]: Starting legacy IPC provider...
[runtime] 2022-07-13 07:22:03 [verbose]: Attempting to use custom storage location /Users/dean/Games/osu-lazer-2
[runtime] 2022-07-13 07:22:03 [verbose]: Storage successfully changed to /Users/dean/Games/osu-lazer-2.
[runtime] 2022-07-13 07:22:05 [verbose]: GL Initialized
```
Until now, migrating would always attempt to move files.
There's a chance that a user is reinstalling osu! but has their data at
a custom location. We want to allow the chance for them to continue
using the external data. This seems like the easiest way to make it
work.
Would be nice if we had a `Game.Restart()` method, but maybe this is
enough for now?
Note that further down the road we will probably prompt the user to
potentially select a custom install path (including one with existing
data) before osu! gets to writing anything.
