Prevent 500 errors if a listing id does not exist

Currently if a user tries to access a page that references a listing that does not exist in the datastore a server error occurs (due to null pointer exception).  This will instead return a 404 not found response if the listing does not exist.
Merge pull request #218 from alphagov/avoid_500s_for_null_listing

Prevent 500 errors if a listing id does not exist