Reveal syntax error in parsing .mention-bot file by leaving a comment (…

…facebookarchive#189)

* Refs facebookarchive#188, Add catch block for syntax error in config file

* Refs facebookarchive#188, Create comment to notify custom config file parsing error

* Refs facebookarchive#188, English improvements of the message

server.js

@@ -163,6 +163,17 @@ async function work(body) {
       headers: {
         Accept: 'application/vnd.github.v3.raw+json'
       }
+    }).catch(function(e) {
+      if (e instanceof SyntaxError && repoConfig.actions.indexOf(data.action) !== -1) {
+        // Syntax error while reading custom configuration file
+        var message =
+          'Unable to parse mention-bot custom configuration file due to a syntax error.\n' +
+          'Please check the potential root causes below:\n\n' +
+          '1. Having comments\n' +
+          '2. Invalid JSON type\n' +
+          '3. Having extra "," in the last JSON attribute';
+        createComment(data, message);
+      }
     });
 
     repoConfig = {...repoConfig, ...configRes};