title: I am getting issue in mongoose calback in custom validation
body: <p>I am using express-validator and have made one custom validation with mongoose database find, now I can't send withMessage. Here is my code althought process stop at Error but it does not show message in Json, but when user created it is show all.</p>
  
  <pre><code>        body('mobile', 'Mobile number is required')
            .custom((value, {req, loc, path}) =&gt; {
              User.countDocuments({mobile: value}, function (err, c) {
                if (err) {
                  console.log('An error happen in db')
                }
                if (c &gt; 0) {
                  throw new Error('Mobile already  exists finally')
                } else {
                  return value
                }
              })
              return value
            })
            .withMessage('Mobile already exists'),
  </code></pre>
  
  <p>Following is log of console</p>
  
  <pre><code>functions: Beginning execution of "app"
  &gt;  events.js:298
  &gt;        throw er; // Unhandled 'error' event
  &gt;        ^
  &gt;
  &gt;  Error: Mobile already  exists finally
  &gt;      at /Users/kamal/Documents/personal/asghar/codes/functions/app/controllers/users_controller.js:117:23
  &gt;      at /Users/kamal/Documents/personal/asghar/codes/functions/node_modules/mongoose/lib/model.js:4849:16
  &gt;      at /Users/kamal/Documents/personal/asghar/codes/functions/node_modules/mongoose/lib/model.js:4849:16
  &gt;      at /Users/kamal/Documents/personal/asghar/codes/functions/node_modules/mongoose/lib/helpers/promiseOrCallback.js:24:16
  &gt;      at /Users/kamal/Documents/personal/asghar/codes/functions/node_modules/mongoose/lib/model.js:4872:21
  &gt;      at /Users/kamal/Documents/personal/asghar/codes/functions/node_modules/mongoose/lib/query.js:4379:11
  &gt;      at /Users/kamal/Documents/personal/asghar/codes/functions/node_modules/kareem/index.js:135:16
  &gt;      at processTicksAndRejections (internal/process/task_queues.js:79:11)
  &gt;  Emitted 'error' event on Function instance at:
  &gt;      at /Users/kamal/Documents/personal/asghar/codes/functions/node_modules/mongoose/lib/model.js:4851:13
  &gt;      at /Users/kamal/Documents/personal/asghar/codes/functions/node_modules/mongoose/lib/helpers/promiseOrCallback.js:24:16
  &gt;      [... lines matching original stack trace ...]
  </code></pre>
  
  <p>I need to add if condition near <code>return value</code> but problem is call back does not bring me value of c here, in above it is comign correct and even stop due to Error raising, but I don't want to raise error instead want to go further <code>withMessage('Mobile already exists')</code> I am sure doing mistake in callback. Please suggest solution</p>