VIM-4419: DRM player error UI

[ghking]

Ticket

https://vimean.atlassian.net/browse/VIM-4419

Ticket Summary

This implements UI states for DRM-specific errors in the player.

Implementation Summary

Since NSErrors from a failed AVAssetResourceLoadingRequest are propagated back to our video player as an underlying error, I added steps to select the appropriate error object in our status observer KVO.

It's kind of a bummer that an NSObject can come from three different places (Who is to say which one we want to forward on?) but this seems like a reasonable selection process.

Associated PR: https://github.vimeows.com/MobileApps/Vimeo-iOS/pull/960

How to Test

See associated PR.

[alfiehanssen]

This jawn assigned?

[alfiehanssen]

NICE DWR 💥