watchlist: Suggest corrections for typos and improve error message consistency.

watchlist: Suggest corrections for typos and improve error message consistency.
https://bugs.webkit.org/show_bug.cgi?id=68976

Reviewed by Adam Barth.

* Scripts/webkitpy/common/watchlist/watchlistparser.py: Add suggestions
for typos and improve consistency of the error messages.
* Scripts/webkitpy/common/watchlist/watchlistparser_unittest.py: Add tests
and improve the regex that were already present.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@96232 268f45cc-cd09-0410-ab3c-d52691b4dbfc