Tweak key used in error JSON for url-arbiter error.

Using an underscored variant will make consumption of this easier.  Will
allow things like `with_indifferent_access` and OpenStruct to work well.
Remove some cruft that was left behind.
Merge pull request #45 from alphagov/tweak_arbiter_error

 Tweak key used in error JSON for url-arbiter error.