Auth error handling for express-jwt

To handle the auth-related errors thrown by express-jwt when it tries to validate JWT tokens in incoming requests, we need to add the following error-catching code to the Express app configuration in mern-skeleton/server/express.js, near the end of the code, after the routes are mounted and before the app is exported: 

app.use((err, req, res, next) => {
  if (err.name === 'UnauthorizedError') {
    res.status(401).json({"error" : err.name + ": " + err.message})
  }
})

express-jwt throws an error named UnauthorizedError when the token cannot be validated for some reason. We catch this error here to return a 401 status back to the requesting client.

With user auth implemented for protecting routes, we have covered all the desired features of a working backend for the skeleton MERN application. In the next section, we will look at how we can check if this standalone backend is functioning as desired without implementing a frontend.