CRM_Core_Error::fatal triggering a php exception when called from a payment extension

Namely, our Sagepay extension has suddenly stopped producing any errors when something goes wrong.

This seems to relate to some recent changes to the top of CRM_Core_Error::fatal().

See also related issue CRM-11042

I'm not able to contribute a fix in this case, as the change is undocumented and poorly commented, so I'm not sure if I would be fixing it in the right way.

Currently, we have put a fix in place in the Sagepay extension to set CRM_Core_Error::$modeException to null before calling CRM_Core_Error::fatal()

See also: http://forum.civicrm.org/index.php/topic,26198.msg111537.html#msg111537

If someone could look into this, that would be great.