Add safeties against exceptions in OnlineLookupCache

[peppy]

Noticed in passing that an exception inside a lookup would cause the cache to never work again, and also not report this in logs at all.

🙈

[bdach]

Logger.Error() will post notifications. Combined with the fact that this cache batches requests in 50 per batch this will potentially get very noisy, and people with bad connections already scream at us for spamming notifications.

I'd recommend switching to

Suggested change

	Logger.Error(t.Exception.AsSingular(), $"{nameof(OnlineLookupCache<TLookup, TValue, TRequest>)} lookup request failed!");
	Logger.Log($@"{nameof(OnlineLookupCache<TLookup, TValue, TRequest>)} lookup request failed!:\n{t.Exception.AsSingular()}", LoggingTarget.Network);

[peppy]

I did check on this and they are debounced, also won't be fired in case of an actual network error (LUCKILY, else this would be game over for most users), see:

osu/osu.Game/Online/API/APIAccess.cs

Lines 371 to 375 in 8bb885a

	catch (Exception e)
	{
	// todo: fix exception handling
	request.Fail(e);
	}

Felt like something we probably want to fire to sentry, but can alter if you disagree.

[bdach]

maybe fine then, not sure 🤷

[bdach]

Could just clear the pending task in the OnlyOnFaulted continuation that was added anyway for logging purposes instead. No real preference here, maybe slightly nicer.