Remove some #try from attributes_for_render

* By the time we're here, we're at a recognised host, which necessarily
  implies an associated site and org
* Some things may still not be there, but those things are limited to:
  * A mapping (implies 404)
  * A TNA timestamp (implies nothing very much, as a default will be generated)
Merge pull request #85 from alphagov/try-there-is-no-try

Remove some #try from attributes_for_render