test: improve assertion message in internet dgram test

When an error occurs unexpectedly, let the user know what the error is.

PR-URL: #30545
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/internet/test-dgram-send-cb-quelches-error.js

@@ -28,7 +28,7 @@ function callbackOnly(err) {
 }
 
 function onEvent(err) {
-  assert.fail('Error should not be emitted if there is callback');
+  assert.fail(`Error should not be emitted if there is callback: ${err}`);
 }
 
 function onError(err) {