Fixes #99 -- Additional assertion, changed error code, wrapped comments

- Assert that errorCount() == 1 before asserting error conditions
- Changed `WEBVTT_INVALID_CUESETTING` to `WEBVTT_MISSING_CUESETTING_KEYWORD` to
  meet the needs of the code. As mentioned in the issue, it's debatable whether
  this is the right way to go about this, as `WEBVTT_INVALID_CUESETTING` makes
  sense as well.
- Wrapped comments to 80 columns
Fixes #99 - Corrected macro name typo.

ASSERT_EQUALS -> ASSERT_EQ
Fixes #99 - Removed extra asterisk from comment and rewrapped
Merge pull request #99 from caitp/issue-99

CueSettingSize.NoKeyword -- Incorrect assertions