Since a few commits, I don't generate the traces anymore in errors.c but
delay this to gui-message.c and rely on the message severity to decide
whether or not generating traces.
Unfortunately none of the current severities are properly describing
this new type of messages. Even GIMP_MESSAGE_ERROR is used everywhere in
our code NOT for actual programming bug, but often for data errors
(which are not bugs but proper messages and should obviously not prompt
a debug trace).