code: function bucketAccessible(s3, bucketName) {
    return new Promise(function(resolve, reject) {
      var params = {
        Bucket: bucketName
      };
      s3.headBucket(params, function(err, data) {
        if (err) {
          reject(new VError(err, 'S3 bucket not accessible'));
        } else {
          resolve();
        }
      });
    });
  }
docs: Return a promise that resolves if the bucket is available and
  rejects if not.
  
  @param {any} s3 S3 client object
  @param {any} bucketName Bucket name
  @returns Promise