Fix bug #6462 (Go to First Error/Warning command broken) - Screen out
header sections (even invisible ones) when looking for first clickable row
in results table.
With unit tests.

Speaking of linting, also fixes two whitespace errors in DocumentCommandHandlers.
Merge pull request #6466 from adobe/pflynn/fix-goto-first-error

Fix bug #6462 (Go to First Error/Warning command broken)