Add exception for serializing unserializable exceptions.
Add exception for serializing unserializable exceptions.

The SimpleSAML_Error_Exception class is a base class for serializable
exceptions. This patch adds SimpleSAML_Error_UnserializableException.
That class saves some of the information from an Exception and saves it
in an serializable exception.

Rud5G/simplesamlphp,Rud5G/simplesamlphp,Rud5G/simplesamlphp,Rud5G/simplesamlphp