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.
vivekrajenderan/simplesamlphp,tectronics/simplesamlphp,vivekrajenderan/simplesamlphp,vivekrajenderan/simplesamlphp,vivekrajenderan/simplesamlphp,tectronics/simplesamlphp,vivekrajenderan/simplesamlphp,tectronics/simplesamlphp,tectronics/simplesamlphp,tectronics/simplesamlphp