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

lib/events.js

@@ -154,10 +154,11 @@ EventEmitter.prototype.emit = function emit(type, ...args) {
 
   // If there is no 'error' event listener then throw.
   if (doError) {
-    let er;
+    let er, stringifiedEr;
     if (args.length > 0)
       er = args[0];
     if (er instanceof Error) {
+      stringifiedEr = er;
       try {
         const { kExpandStackSymbol } = require('internal/util');
         const capture = {};
@@ -171,10 +172,17 @@ EventEmitter.prototype.emit = function emit(type, ...args) {
       // Note: The comments on the `throw` lines are intentional, they show
       // up in Node's output if this results in an unhandled exception.
       throw er; // Unhandled 'error' event
+    } else {
+      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();
 
@@ -9,7 +10,7 @@ common.expectsError(
   {
     code: 'ERR_UNHANDLED_ERROR',
     type: Error,
-    message: 'Unhandled error. (Accepts a string)'
+    message: "Unhandled error. ('Accepts a string')"
   }
 );
 
@@ -18,6 +19,18 @@ common.expectsError(
   {
     code: 'ERR_UNHANDLED_ERROR',
     type: Error,
-    message: 'Unhandled error. ([object Object])'
+    message: "Unhandled error. ({ message: 'Error!' })"
+  }
+);
+
+common.expectsError(
+  () => EE.emit('error', {
+    message: 'Error!',
+    [util.inspect.custom]() { throw null; }
+  }),
+  {
+    code: 'ERR_UNHANDLED_ERROR',
+    type: Error,
+    message: "Unhandled error. ([object Object])"
   }
 );