Added a comment to app.js about default express error handler behaviour.

waspc/data/Generator/templates/server/src/app.js

@@ -22,6 +22,14 @@ app.use((err, _req, res, next) => {
     return res.status(err.statusCode).json({ message: err.message, data: err.data })
   }
 
+  // This forwards the error to the default express error handler.
+  // As described by expressjs documentation, the default error handler sets response status
+  // to err.status or err.statusCode if it is 4xx or 5xx, and if not, sets it to 500.
+  // It won't add any more info to it if server is running in production, which is exactly what we want,
+  // we want to share as little info as possible when error happens in production, for security reasons,
+  // so they will get only status code if set, or 500 if not, no extra info.
+  // In development it will also share the error stack though, which is useful.
+  // If the user wants to put more information about the error into the response, they should use HttpError.
   return next(err)
 })