Allow use of a predicate function for validation of thrown exception

We had a case where the validation of our exception was more than just a comparison on the error.message parameter.  

We need to validate that our methods threw a specific exception type on the following spec:
http://specs.wacapps.net/wac1_0/dec2010/exception.html

Since error.type is not a standard field (like Microsoft's error.description or Mozilla's error.lineNumber) we didn't want to add a check natively into the test runner.

The most extensible way would be to allow us to use a predicate function when validating exceptions to allow for people to test for browser or runtime specific fields on the error.