events: update and clarify error message

Update error message that's thrown when no error listeners are attached
to an emitter.

PR-URL: https://github.com/nodejs/node/pull/10387
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

lib/events.js

@@ -171,7 +171,7 @@ EventEmitter.prototype.emit = function emit(type) {
       er = arguments[1];
     if (domain) {
       if (!er)
-        er = new Error('Uncaught, unspecified "error" event');
+        er = new Error('Unhandled "error" event');
       if (typeof er === 'object' && er !== null) {
         er.domainEmitter = this;
         er.domain = domain;
@@ -182,7 +182,7 @@ EventEmitter.prototype.emit = function emit(type) {
       throw er; // Unhandled 'error' event
     } else {
       // At least give some kind of context to the user
-      var err = new Error('Uncaught, unspecified "error" event. (' + er + ')');
+      const err = new Error('Unhandled "error" event. (' + er + ')');
       err.context = er;
       throw err;
     }

test/parallel/test-event-emitter-errors.js

@@ -5,6 +5,10 @@ const assert = require('assert');
 
 const EE = new EventEmitter();
 
-assert.throws(function() {
+assert.throws(() => {
   EE.emit('error', 'Accepts a string');
-}, /Accepts a string/);
+}, /^Error: Unhandled "error" event\. \(Accepts a string\)$/);
+
+assert.throws(() => {
+  EE.emit('error', {message: 'Error!'});
+}, /^Error: Unhandled "error" event\. \(\[object Object\]\)$/);