CASC-233 ErrorRedirectFilter Can throw Null Pointer Exception

Problem: the ErrorRedirectFilter assumes there is a root cause and will null pointer if there is not.
Solution: Only use the root cause if there is one, otherwise use the original exception.

QA Notes: Added unit test to confirm non-root cause error.
I always find excessive a ```catch (Throwable```. A catch on ```Exception``` was not enough?