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.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@1564 44740490-163a-0410-bde0-09ae8108e29a