Uses the ELB.describeLoadBalancers method to check the existence of a load
balancer. If the list of LoadBalancerDescriptions returned by the request
is empty, doesLoadBalancerExist is resolved as False, if it's not empty it
is resolved as True.

@param elb
@param loadBalancerName
@returns {Promise}
function doesLoadBalancerExist (elb, loadBalancerName) {
  function inspectError (err) {
    if (err.code === 'LoadBalancerNotFound') {
      return false
    }

    return Bluebird.reject(err)
  }

  return elb
    .describeLoadBalancersPromised({
      LoadBalancerNames: [ loadBalancerName ]
    })
    .then(_.constant(true))
    .error(inspectError)
}