events: show inspected error in uncaught 'error' message

If there is no handler for `.emit('error', value)` and `value`
is not an `Error` object, we currently just call `.toString()`
on it.

Almost always, using `util.inspect()` provides better information
for diagnostic purposes, so prefer to use that instead.

Refs: nodejs/help#1729

PR-URL: #25621
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Matheus Marchini <mat@mmarchini.me>

lib/events.js

@@ -172,9 +172,18 @@ EventEmitter.prototype.emit = function emit(type, ...args) {
       // up in Node's output if this results in an unhandled exception.
       throw er; // Unhandled 'error' event
     }
+
+    let stringifiedEr;
+    const { inspect } = require('internal/util/inspect');
+    try {
+      stringifiedEr = inspect(er);
+    } catch {
+      stringifiedEr = er;
+    }
+
     // At least give some kind of context to the user
     const errors = lazyErrors();
-    const err = new errors.ERR_UNHANDLED_ERROR(er);
+    const err = new errors.ERR_UNHANDLED_ERROR(stringifiedEr);
     err.context = er;
     throw err; // Unhandled 'error' event
   }

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

@@ -1,6 +1,7 @@
 'use strict';
 const common = require('../common');
 const EventEmitter = require('events');
+const util = require('util');
 
 const EE = new EventEmitter();
 
@@ -9,12 +10,24 @@ common.expectsError(
   {
     code: 'ERR_UNHANDLED_ERROR',
     type: Error,
-    message: 'Unhandled error. (Accepts a string)'
+    message: "Unhandled error. ('Accepts a string')"
   }
 );
 
 common.expectsError(
   () => EE.emit('error', { message: 'Error!' }),
+  {
+    code: 'ERR_UNHANDLED_ERROR',
+    type: Error,
+    message: "Unhandled error. ({ message: 'Error!' })"
+  }
+);
+
+common.expectsError(
+  () => EE.emit('error', {
+    message: 'Error!',
+    [util.inspect.custom]() { throw new Error(); }
+  }),
   {
     code: 'ERR_UNHANDLED_ERROR',
     type: Error,