Throw exception when trying to invalidate FakeHttpSession

The FakeHttpSession is used only for Tomcat 7 as far as I can see (#11721)
Invalidating the fake session will not invalidate the real session so
the end result will not be what the user expects, therefore we throw an
exception instead in this case.

Change-Id: I1703644736d81ee2870b709517cbe5fa523d00f7