feat(LockCachingAudioSource): add network error handling

[danielgomezriconeon]

Try to fix:

• An Unhandled Exception is thrown when loading LockCachingAudioSource fails. #1213
• LockCachingAudioSource does not recover from network errors #594

Done:

• Add retry mechanism for _fetch
• Do not throw exceptions on query but return Future with errors so caller could handle them gracefully

[danielgomezriconeon]

@ryanheise I'm having troubles trying to test multiple scenarios, can you please give me a hint?

I added a retry logic on the fetch method, but that's running without awaiting it.

And for some reason, if an exception is thrown there or the future returns an error, it is not being propagated to the main await, where:
https://github.com/ryanheise/just_audio/pull/1398/files#diff-1425e2b963b2b8d4c9fde51b8fabf85b624477e9373c365a72bea2aa6071990bR137-R143