Deleting a firmware signing key that has associations produces large error

We need to capture constraint errors and present them in a pretty way.
Deleting remote signing key test
Unhandled error: {:error, {:error, %Jason.DecodeError{data: "# Ecto.ConstraintError at DELETE /orgs/nerveshub/keys/test\n\nException:\n\n    ** (Ecto.ConstraintError) constraint error when attempting to delete struct:\n    \n        * foreign_key: firmwares_tenant_key_id_fkey\n    \n    If you would like to convert this constraint into an error, please\n    call foreign_key_constraint/3 in your changeset and define the proper\n    constraint name. The changeset has not defined any constraint.\n    \n        (ecto) lib/ecto/repo/schema.ex:574: anonymous fn/4 in Ecto.Repo.Schema.constraints_to_errors/3\n        (elixir) lib/enum.ex:1314: Enum.\"-map/2-lists^map/1-0-\"/2\n        (ecto) lib/ecto/repo/schema.ex:559: Ecto.Repo.Schema.constraints_to_errors/3\n        (ecto) lib/ecto/repo/schema.ex:386: anonymous fn/9 in Ecto.Repo.Schema.do_delete/4\n        (ecto) lib/ecto/repo/schema.ex:774: anonymous fn/3 in Ecto.Repo.Schema.wrap_in_transaction/6\n        (ecto) lib/ecto/adapters/sql.ex:576: anonymous fn/3 in Ecto.Adapters.SQL.do_transaction/3\n        (db_connection) lib/db_connection.ex:1283: DBConnection.transaction_run/4\n        (db_connection) lib/db_connection.ex:1207: DBConnection.run_begin/3\n        (db_connection) lib/db_connection.ex:798: DBConnection.transaction/3\n        (nerves_hub_api) lib/nerves_hub_api_web/controllers/key_controller.ex:34: NervesHubAPIWeb.KeyController.delete/2\n        (nerves_hub_api) lib/nerves_hub_api_web/controllers/key_controller.ex:1: NervesHubAPIWeb.KeyController.action/2\n        (nerves_hub_api) lib/nerves_hub_api_web/controllers/key_controller.ex:1: NervesHubAPIWeb.KeyController.phoenix_controller_pipeline/2\n        (nerves_hub_api) lib/nerves_hub_api_web/endpoint.ex:1: NervesHubAPIWeb.Endpoint.instrument/4\n        (phoenix) lib/phoenix/router.ex:275: Phoenix.Router.__call__/1\n        (nerves_hub_api) lib/nerves_hub_api_web/endpoint.ex:1: NervesHubAPIWeb.Endpoint.plug_builder_call/2\n        (nerves_hub_api) lib/plug/debugger.ex:122: NervesHubAPIWeb.Endpoint.\"call (overridable 3)\"/2\n        (nerves_hub_api) lib/nerves_hub_api_web/endpoint.ex:1: NervesHubAPIWeb.Endpoint.call/2\n        (phoenix) lib/phoenix/endpoint/cowboy2_handler.ex:33: Phoenix.Endpoint.Cowboy2Handler.init/2\n        (cowboy) /Users/jschneck/Developer/nerves/nerves_hub_web/deps/cowboy/src/cowboy_handler.erl:37: :cowboy_handler.execute/2\n        (cowboy) /Users/jschneck/Developer/nerves/nerves_hub_web/deps/cowboy/src/cowboy_stream_h.erl:274: :cowboy_stream_h.execute/3\n    \n\n## Connection details\n\n### Params\n\n    %{\"name\" => \"test\", \"org_name\" => \"nerveshub\"}\n\n### Request info\n\n  * URI: https://0.0.0.0:4002/orgs/nerveshub/keys/test\n  * Query string: \n\n### Headers\n  \n  * content-type: application/json\n  * host: 0.0.0.0:4002\n  * user-agent: hackney/1.13.0\n\n### Session\n\n    %{}\n", position: 0, token: nil}}}
nerves-hub / nerves_hub_cli

Deleting a firmware signing key that has associations produces large error #23
