doc: handle pending exception in callback wrapper

fixes: nodejs/node-addon-api#1025

The functionreference test from the node-api tests
was reporting a failed v8 check when Node.js was compiled
as debug. The failure was because an exception was
pending but the C++ wrappers were returning
a return value that was invalid.

Signed-off-by: Michael Dawson <mdawson@devrus.com>

src/js_native_api_v8.cc

@@ -306,12 +306,16 @@ class CallbackWrapperBase : public CallbackWrapper {
     napi_env env = _bundle->env;
     napi_callback cb = _bundle->cb;
 
-    napi_value result;
+    napi_value result = nullptr;
+    bool exceptionOccurred = false;
     env->CallIntoModule([&](napi_env env) {
       result = cb(env, cbinfo_wrapper);
+    }, [&](napi_env env, v8::Local<v8::Value> value) {
+      exceptionOccurred = true;
+      env->isolate->ThrowException(value);
     });
 
-    if (result != nullptr) {
+    if (!exceptionOccurred && (result != nullptr)) {
       this->SetReturnValue(result);
     }
   }