Error Handling Middleware

Summary

A special 4-argument Express middleware (`(err, req, res, next)`) that catches errors thrown anywhere in your route handlers and formats a consistent error response.

• 1Add a global error-handling middleware as the last piece of your middleware stack.
• 2Throw a custom error from a route and confirm it's caught and formatted correctly.
• 3Learn why error middleware must be registered after all your routes.