Differentiate ErrorException from GracefullTerminationException

[lewart]

In our application we initialize the CAS Client together with calling CAS_GracefullTerminationException::throwInsteadOfExiting() method and then we handle exceptions in our own fashion.

Right now the same exception CAS_GracefullTerminationException is used when the library ends its work and when there is a real error reported by a phpCAS::error() method.

I think it would be helpful for the integrators, to use a different exception for errors reported in the phpCAS::error() method.

[jfritschi]

Sounds like a good idea. We will be happy to receive a pull request if you already have a patch in place ...