While CoglError is a define to GError, it doesn't follow the convention
of ignoring errors when NULL is passed, but rather treats the error as
fatal :-(
That's clearly unwanted for a compositor, so make sure to always pass
an error parameter where a runtime error is possible (i.e. any CoglError
that is not a malformed blend string).
https://bugzilla.gnome.org/show_bug.cgi?id=765058