Sending a nonexistent id when creating or updating a resource via API ends up with a foreign key exception on foreman instances running on a postgres backend.

Reproducer using hammer:

hammer domain create --dns-id 3283 --name test.org
Could not create the domain:
  ERROR:  insert or update on table "domains" violates foreign key constraint "domains_dns_id_fk" 
  DETAIL:  Key (dns_id)=(3283) is not present in table "smart_proxies".

Relevant part of the server log:

2016-11-30T08:09:17 8a771c14 [app] [I] Started POST "/api/domains" for fe80::5054:ff:fef5:1b57 at 2016-11-30 08:09:17 +0000
2016-11-30T08:09:17 8a771c14 [app] [I] Processing by Api::V2::DomainsController#create as JSON
2016-11-30T08:09:17 8a771c14 [app] [I]   Parameters: {"domain"=>{"name"=>"test.org", "dns_id"=>"3283"}, "apiv"=>"v2"}
2016-11-30T08:09:17 8a771c14 [app] [W] Action failed
 | PG::ForeignKeyViolation: ERROR:  insert or update on table "domains" violates foreign key constraint "domains_dns_id_fk" 
 | DETAIL:  Key (dns_id)=(3283) is not present in table "smart_proxies".
 |

...

2016-11-30T08:09:17 8a771c14 [app] [I]   Rendered api/v2/errors/standard_error.json.rabl within api/v2/layouts/error_layout (0.5ms)
2016-11-30T08:09:17 8a771c14 [app] [I] Completed 500 Internal Server Error in 19ms (Views: 1.9ms | ActiveRecord: 2.4ms)

The reproducer uses domains as a simple example, but the problem is present across all resources in the API.

Expected results:
The server should reply with a more user friendly message instead, for example:
Proxy (id: 3283) doesn't exist.

Additional INFO:
I haven't tested the behavior on different database backends.