Display jsonlint error in parsing .mention-bot file with syntax error (…

…#190)

* Refs #188, add jsonlint library

* Refs #188, display jsonlint error log in comment as code block

package.json

@@ -32,6 +32,7 @@
     "flow-bin": "^0.18.1",
     "github": "^2.1.0",
     "jest-cli": "^0.7.1",
+    "jsonlint": "^1.6.2",
     "minimatch": "^3.0.0",
     "node-schedule": "^1.0.0"
   }

server.js

@@ -19,6 +19,7 @@ var util = require('util');
 var schedule = require('./schedule.js');
 
 var GitHubApi = require('github');
+var jsonlint = require('jsonlint');
 
 var CONFIG_PATH = '.mention-bot';
 
@@ -103,6 +104,7 @@ function getRepoConfig(request) {
         var data = JSON.parse(result.data);
         resolve(data);
       } catch (e) {
+        e.repoConfig = result.data;
         reject(e);
       }
     });
@@ -166,12 +168,20 @@ async function work(body) {
     }).catch(function(e) {
       if (e instanceof SyntaxError && repoConfig.actions.indexOf(data.action) !== -1) {
         // Syntax error while reading custom configuration file
+        var errorLog = '';
+        try {
+          jsonlint.parse(e.repoConfig)
+        } catch(err) {
+          errorLog = err;
+        }
         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';
+          '3. Having extra "," in the last JSON attribute\n\n' +
+          'Error message:\n' +
+          '```\n' + errorLog + '\n```';
         createComment(data, message);
       }
     });