vm: don't print out arrow message for custom error

In `AppendExceptionLine()`, which is used both by the `vm`
module and the uncaught exception handler, don’t print anything
to stderr when called from the `vm` module, even if the
thrown object is not a native error instance.

Fixes: nodejs#7397

src/node.cc

@@ -1423,7 +1423,8 @@ bool IsExceptionDecorated(Environment* env, Local<Value> er) {
 
 void AppendExceptionLine(Environment* env,
                          Local<Value> er,
-                         Local<Message> message) {
+                         Local<Message> message,
+                         bool handlingFatalError) {
   if (message.IsEmpty())
     return;
 
@@ -1510,15 +1511,16 @@ void AppendExceptionLine(Environment* env,
 
   Local<String> arrow_str = String::NewFromUtf8(env->isolate(), arrow);
 
-  if (!arrow_str.IsEmpty() && !err_obj.IsEmpty() && err_obj->IsNativeError()) {
+  if (!arrow_str.IsEmpty() && !err_obj.IsEmpty() &&
+      (!handlingFatalError || err_obj->IsNativeError())) {
     err_obj->SetPrivate(
         env->context(),
         env->arrow_message_private_symbol(),
         arrow_str);
     return;
   }
 
-  // Allocation failed, just print it out.
+  // Allocation failed when called from ReportException(), just print it out.
   if (env->printed_error())
     return;
   env->set_printed_error(true);
@@ -1532,7 +1534,7 @@ static void ReportException(Environment* env,
                             Local<Message> message) {
   HandleScope scope(env->isolate());
 
-  AppendExceptionLine(env, er, message);
+  AppendExceptionLine(env, er, message, true);
 
   Local<Value> trace_value;
   Local<Value> arrow;

src/node_internals.h

@@ -139,7 +139,8 @@ bool IsExceptionDecorated(Environment* env, v8::Local<v8::Value> er);
 
 void AppendExceptionLine(Environment* env,
                          v8::Local<v8::Value> er,
-                         v8::Local<v8::Message> message);
+                         v8::Local<v8::Message> message,
+                         bool handlingFatalError = false);
 
 NO_RETURN void FatalError(const char* location, const char* message);
 

test/message/vm_caught_custom_runtime_error.js

@@ -0,0 +1,17 @@
+'use strict';
+require('../common');
+const vm = require('vm');
+
+console.error('beginning');
+
+// Regression test for https://github.com/nodejs/node/issues/7397:
+// This should not print out anything to stderr due to the error being caught.
+try {
+  vm.runInThisContext(`throw ({
+    name: 'MyCustomError',
+    message: 'This is a custom message'
+  })`, { filename: 'test.vm' });
+} catch (e) {
+}
+
+console.error('end');

test/message/vm_caught_custom_runtime_error.out

@@ -0,0 +1,2 @@
+beginning
+end