do not use exceptions for code flow

I have to admit that I personally would not even care about this error condition, as I'd expect my errors to listen to :identifier

lib/api/v3/errors/error_representer.rb

@@ -57,7 +57,7 @@ module API
         def error_identifier
           return 'urn:openproject-org:api:v3:errors:MultipleErrors' unless Array(represented.errors).empty?
 
-          represented.class.identifier rescue nil
+          represented.class.identifier if represented.class.respond_to?(:identifier)
         end
       end
     end