Update error handler to give an JSON always

[vishwaefor]

Steps

update app.js

// error handler
app.use(function (err, req, res, next) {
  // set locals, only providing error in development
  res.locals.message = err.message;
  res.locals.error = req.app.get('env') === 'development' ? err : {};

  // JSON error
  res.status(err.status || 500);
  res.json({ errors: [{ msg: err.message }] }); // change render to json method
});