Original commit message from CVS:
* gst/pygstexception.c: (element_not_found_error_init),
(pygst_exceptions_register_classes):
* gst/pygstexception.h:
gst.element_factory_make should raise ElementNotFoundError.
Subclass it from PluginNotFoundError so we can add it compatibly
and remove the wrong one later.
* gst/gstelementfactory.override:
raise ElementNotFoundError