While all other resources are supposed to be already loaded, this is not
the case of fonts which are loaded in a thread, and whose loading may
end after plug-ins are initialized. So ignore unknown font name as an
exception (because we can't know if it is really unknown or just not
loaded yet).
This fixes such WARNINGs:
> (script-fu:33006): scriptfu-WARNING **: 14:35:28.259: script_fu_add_resource_arg declared resource name is invalid Serif
51d69fa44b