Add a new hook for exception handling.
Add a new hook for exception handling.

In line with the previous commit, we now allow hooking in the exception handler, so that a module can implement its own logic to deal with certain exceptions.

sitya/simplesamlphp,dnmvisser/simplesamlphp,ghalse/simplesamlphp,simplesamlphp/simplesamlphp,simplesamlphp/simplesamlphp,ghalse/simplesamlphp,sitya/simplesamlphp,jschlyter/simplesamlphp,dnmvisser/simplesamlphp,jschlyter/simplesamlphp,ghalse/simplesamlphp,simplesamlphp/simplesamlphp,ghalse/simplesamlphp,dnmvisser/simplesamlphp,jschlyter/simplesamlphp,jschlyter/simplesamlphp,dnmvisser/simplesamlphp,sitya/simplesamlphp,jschlyter/simplesamlphp,simplesamlphp/simplesamlphp,sitya/simplesamlphp