Return Validation Error Messages

If the errors array contains any elements (indicating that one of our validation rules failed) let's return a response with a 400 Bad Request HTTP status code along with the errors.

1. In route.js, add the if statement shown below to the POST /api/users route handler:

   // Validate that we have a `name` value.
   if (!user.name) { ... }
   
   // Validate ...