Issue 388: HandlingResult.getException() throws exp if none thrown.

When calling HandlingResult.getException() during a unit test and no
exception was thrown, the method throws a
HandlerExceptionNotThrownException instead of returning null.
Issue 388 - Pass expected exception type to HandlingResult.exception(c)

Rename method from getException() to exception(Class<T> type). Methods
throws exception if the type throw is not the expected type passed to
the method.
Issue 388: Throw exception when accessing handle results after excep.
Merge pull request #473 from johnrengelman/handling-result-excep

Issue 388: Improved error handling in HandlingResult.