Clean up error messages.
Partly fix #32:

_getHtmlAsString returned on error, rather than deferring. When there were around sixteen errors (typically timeouts), the process exited.
Add better error output, refactor.
Add better error output, refactor pt. 2.
Merge branch 'master' of https://github.com/operasoftware/ucss
Replace 'nosummary' mode with 'silent' mode.