deleting a group can cause a constraint error

[mpeck]

When deleting a group Ecto with sometimes throw a constraint error:

2017-01-30T16:19:53.891 [error] #PID<0.2817.0> running Cog.Endpoint terminated
Server: localhost:4000 (http)
Request: DELETE /v1/groups/589561a5-3391-41f1-91d8-896e8e0f2a62
** (exit) an exception was raised:
    ** (Ecto.ConstraintError) constraint error when attempting to delete struct:

    * foreign_key: user_group_membership_group_id_fkey

If you would like to convert this constraint into an error, please
call foreign_key_constraint/3 in your changeset and define the proper
constraint name. The changeset has not defined any constraint.

        (ecto) lib/ecto/repo/schema.ex:417: anonymous fn/4 in Ecto.Repo.Schema.constraints_to_errors/3
        (elixir) lib/enum.ex:1184: Enum."-map/2-lists^map/1-0-"/2
        (ecto) lib/ecto/repo/schema.ex:403: Ecto.Repo.Schema.constraints_to_errors/3
        (ecto) lib/ecto/repo/schema.ex:347: anonymous fn/9 in Ecto.Repo.Schema.do_delete/4
        (ecto) lib/ecto/repo/schema.ex:608: anonymous fn/3 in Ecto.Repo.Schema.wrap_in_transaction/6
        (ecto) lib/ecto/adapters/sql.ex:480: anonymous fn/3 in Ecto.Adapters.SQL.do_transaction/3
        (db_connection) lib/db_connection.ex:1063: DBConnection.transaction_run/4
        (db_connection) lib/db_connection.ex:987: DBConnection.run_begin/3

[vanstee]

We'll need to delete a Changeset rather than a Group struct in the Groups repository.