Warn when pool_size is 1 (Ecto 2.0)

[Gazler]

When the pool_size is 1, and the test_helper has something like:

Mix.Task.run "ecto.migrate", ["-r", "MyApp.Repo", "--quiet"]

The migrate task checks out the only pool and holds on to it causing the following error:

** (MatchError) no match of right hand side value: {:error, %DBConnection.Error{message: "failed to checkout using Ecto.Adapters.SQL.Sandbox.Pool"}}

13:22:05.497 [error] GenServer #PID<0.271.0> terminating
** (stop) exited in: :gen_server.call(#PID<0.233.0>, {:checkout, #Reference<0.0.7.2799>, true}, 5000)
    ** (EXIT) time out
    (stdlib) gen_server.erl:212: :gen_server.call/3
    (poolboy) src/poolboy.erl:55: :poolboy.checkout/3
    (db_connection) lib/db_connection/poolboy.ex:36: DBConnection.Poolboy.checkout/2
    (ecto) lib/ecto/adapters/sql/sandbox.ex:214: Ecto.Adapters.SQL.Sandbox.Pool.checkout/2
    (db_connection) lib/db_connection/ownership/owner.ex:83: DBConnection.Ownership.Owner.handle_call/3
    (stdlib) gen_server.erl:629: :gen_server.try_handle_call/4
    (stdlib) gen_server.erl:661: :gen_server.handle_msg/5
    (stdlib) proc_lib.erl:240: :proc_lib.init_p_do_apply/3

We should warn in this case.

Removing pool_size from the config fixes this.

/cc @josevalim

[josevalim]

Fixed in master.