v0.9.1 - only fatal errors should trigger an error event

As part of 0.7.0, tighter error handling integration with hls.js made all errors be triggered on the player. This meant that errors that don't inhibit playback and that hls.js handled automatically were treated the same as fatal errors that hls.js doesn't handle automatically. Now, only errors that hls.js considers fatal will trigger an error event.