Write exception class, for better control.
(When I backport this test to Tomcat 7 the message in this exception is "null", but in Tomcat 8 it is some text. So let's see that what the exception class is).

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1634258 13f79535-47bb-0310-9956-ffa450edef68