Fix missing clause in mysql storage_down/1 (#445)

add case clause in mysql storage_down function to handle all adapter
errors (i.e. caused by an unreachable database)

Author: flaviogrossi

lib/ecto/adapters/myxql.ex

@@ -210,6 +210,8 @@ defmodule Ecto.Adapters.MyXQL do
         {:error, :already_down}
       {:error, %{mysql: %{name: :ER_BAD_DB_ERROR}}} ->
         {:error, :already_down}
+      {:error, error} ->
+        {:error, Exception.message(error)}
       {:exit, :killed} ->
         {:error, :already_down}
       {:exit, exit} ->