due to the fix for #547 backtrace is not always defined
Add cli test for step throwing exception. Unfortuately there's another bug now reporting extra test failures
undo double error event emitting from  modules/casper.js@76b7f73 and the extra coupling in the onError handler
merge from upstream
Merge pull request #673 from nathanboktae/step-exception-bug

Exceptions thrown in steps causes casperjs to again throw and crash