Comment #6 on issue 266 by pekka.klarck: Keyword execution error without failing test case (like Selenium verify)
http://code.google.com/p/robotframework/issues/detail?id=266
This support is now built in to the framework and you should not use the patch attached here. Instead use the approach explained in issue 137 (see comment 20) and documented also in the UserGuide.
