Fixes #245 - Don't return MISSING_CUESETTING

Now skipping to next non-whitespace for all errors related to not
finding a valid keyword

Removed check for 'COLON', all non-valid stating characters other than
whitespace are checked now.
Merge pull request #245 from caitp/issue-245

Should return WEBVTT_INVALID_CUESETTING rather than WEBVTT_MISSING_CUESETTING_KEYWORD