Additional webvtt_status values for use by internal parser functions

Some routines will need to be able to hint to their caller that it should continue on with the buffer, or abort parsing a buffer (without necessarily being a fatal error), or that it has reached its ending correctly.

In my massive pull request, I've created 2 values, `WEBVTT_CONTINUE_LINE` and `WEBVTT_FINISH_LINE`, which are not particularly aptly named.

Add additional errors that are named better and could be used in a similar way.