added ability to override error-controller error handling via HlsConfig

[krseager]

This PR will allow for error-controller to defer handling of errors via HlsConfig.

Why is this Pull Request needed?

In certain cases, you may not want hls.js to raise a fatal when an error occurs OR you would like to handle an error manually. An example would be if WebVTT subtitles are in use and the player enters an adbreak (where no subtitle's exist) OR there is a CDN issue where WebVTT files are unavailable, the developer may want playback to continue rather than raising a fatal error.

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

Ensure that callback functions & documentation is clear.

Resolves issues:

N/A => I will create an issue depending on feedback to this draft PR.

Checklist

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