message: Return Postcode.find's callback to prevent next being called twice
diff: @@ -17,7 +17,9 @@ exports.show = (request, response, next) => {
  ScottishPostcode.find(postcode, (error, result) => {
  if (error) return next(error);
  if (!result) {
  - Postcode.find(postcode, (gerr, gres) => {
  + // The return value of Postcode.find is the return value of the callback
  + // see postcode.js lines 186-188
  + return Postcode.find(postcode, (gerr, gres) => {
  if (gerr) return next(gerr);
  if (!gres) return next(new PostcodeNotFoundError());
  return next(new PostcodeNotInSpdError());
  