Fixes #119 - Corrected assertion, added safety assertion, wraped lines

- Assert error is WEBVTT_INVALID_CUESETTING rather than deprecated
  WEBVTT_MISSING_CUESETTING_KEYWORD
- Assert error count is 1 before asserting error conditions
- Wrap lines to 80 columns
Fixes #119 - Linewrapped comments
Fixes #119 - Replaced `MISSING` with `INVALID` in comment
Merge pull request #119 from caitp/issue-119

CueSettingPosition.NoKeyword incorrect assertions