feat: use hardcoded error message when error class has a nil message

Author: bethesque

lib/pact_broker/api/resources/error_handler.rb

@@ -29,7 +29,7 @@ def self.reportable?(e)
 
         def self.display_message(e, error_reference)
           if PactBroker.configuration.show_backtrace_in_error_response?
-            e.message
+            e.message || obfuscated_error_message(error_reference)
           else
            reportable?(e) ? obfuscated_error_message(error_reference) : e.message
           end