How to avoid cascading promises and catchs?

I'm trying to make an API using node and express. This is the function that creates the user.

I'm not sure if I'm handling errors properly, I feel like being in a "promise hell" because of the async mongodb functions. The next thing I'm going to do is to get the inserted user's id, and I guess it's going to be another promise, another error to handle...

  exports.create = function(req, res, next) {
  var errors = [];
  var userData = req.body;

  // exit if the user didn't fill all fields
  var requiredFields = ['first_name',
                        'last_name',
                        'login',
                        'email',
                        'password',
                        'sex'];
  requiredFields.forEach(function(elem) {
    if (!userData.hasOwnProperty(elem))
      errors.push('The field \'' + elem + '\' is missing');
  });
  if (errors.length !== 0)
    res.status(400).json({errors: errors});

  // check if the user or the login are already in use
  db.connection.collection(COLLECTION_NAME).findOne({ $or: [
                              { email: userData.email },
                              { login: userData.login }
                            ]})
    .then(function(data) {
      // if there is no user (null) we can create it
      if (data === null) {
        db.collection(COLLECTION_NAME).insertOne(userData).then(function (data) {
          res.status(201).json("success");
        }, function (err) {
          res.status(400).json({errors: ["DB error: cannot create user"]});
        })
      } else {
        errors.push('An user is already registered with this email or this login.');
        if (errors.length !== 0)
          res.status(400).json({errors: errors});
      }
    }, function (err) {
      res.status(400).json({errors: errors});
    })
}

Is there a best way to do this ?

By the way, I can't use a validation library nor mongoose.

Thanks.

via raph77777