After the recent changes introducing cancellation support to
`WorkingBeatmap`, it turned out that if the cancellation support was
used, `GetPlayableBeatmap()` would raise timeout exceptions rather than
the expected `OperationCanceledException`.
To that end, split off a separate overload for the typical usage, that
catches `OperationCanceledException` and converts them to beatmap load
timeout exceptions, and use normal `OperationCanceledException`s in the
overload that requires a cancellation token to work.
The recent changes related to adding support for working beatmap load
cancellation exposed a flaw in the beatmap difficulty cache. With the
way the difficulty computation logic was written, any error in the
calculation process (including beatmap load timeout, or cancellation)
would result in a 0.00 star rating being permanently cached in memory
for the given beatmap.
To resolve, change the difficulty cache's return type to nullable.
In failure scenarios, `null` is returned, rather than
`default(StarDifficulty)` as done previously.