Should return WEBVTT_INVALID_CUESETTING rather than WEBVTT_MISSING_CUESETTING_KEYWORD

Missing cuesetting keyword is silly, we should just use the INVALID_CUESETTING error because we're expecting a keyword and not getting one. Will simplify the parser a bit.

parser.c:699 Should return WEBVTT_INVALID_CUESETTING instead.

Additionally, the parser needs to skip the rest of whatever string it ran into that began with a colon. The simple fix is to simply remove the COLON case.