add JSON response for errors when using JS

lib/middlewares.js

@@ -1,7 +1,19 @@
 const FBURLError = () => new Error('Not a valid Facebook URL!')
 
+const sendJSON = (req) => {
+  return req.accepts().includes('application/json')
+}
+
 const genericErrorHandler = (err, req, res, next) => {
   console.error(err.stack)
+
+  if (sendJSON(req)) {
+    res
+      .status(500)
+      .send({ error: err.toString() })
+    return
+  }
+
   res
     .status(500)
     .render('error', { error: err.toString() })