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.
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