Fragment 4xx network error handling improvements

[robwalch]

This PR will...

• Skip errored fragments with no alternate
• Fix fragment retry delay when there are no alternates

Why is this Pull Request needed?

Error handling of fragment 4xx errors skips retry, expecting a switch to an alternate selection or pathway. When there is none, the error was reset, but then the stream controller would request the fragment again on next IDLE tick.

This change skips fragments that 4xx when there are no alternate pathways or media options, treating them as gaps, unless the segment is the last segment in a VOD playlist (#5153, #6171). An init segment or the last segment in a playlist will be retried using retry rules even if the network error is a 4xx (#5647, #6741).

Are there any points in the code the reviewer needs to double check?

Resolves issues:

• Resolves Playback stops after failed retries with 403 #6741
• Resolves Ignore fragLoadPolicy and levelLoadPolicy on livestreams when 404 error #5647
• Resolves Video will not play if 404 on first fragment #5153
• Closes/Replaces [DRAFT] Skip over segments that fail to reload (by applying GAP tags) #6171

Checklist

• changes have been done against master branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• API or design changes are documented in API.md