Creating user via API give correct responses if the user already exists or the mail address is malformed.

lib/beet_round_server_web/controllers/user_json.ex

@@ -15,10 +15,31 @@ defmodule BeetRoundServerWeb.UserJSON do
     %{data: data(user)}
   end
 
+  def show(%{changeset: changeset}) do
+    # When encoded, the changeset returns its errors
+    # as a JSON object. So we just pass it forward.
+    %{errors: Ecto.Changeset.traverse_errors(changeset, &translate_error/1)}
+  end
+
   defp data(%User{} = user) do
     %{
       id: user.id,
       email: user.email
     }
   end
+
+  defp translate_error({msg, opts}) do
+    # You can make use of gettext to translate error messages by
+    # uncommenting and adjusting the following code:
+
+    # if count = opts[:count] do
+    #   Gettext.dngettext(BeetRoundServerWeb.Gettext, "errors", msg, msg, count, opts)
+    # else
+    #   Gettext.dgettext(BeetRoundServerWeb.Gettext, "errors", msg, opts)
+    # end
+
+    Enum.reduce(opts, msg, fn {key, value}, acc ->
+      String.replace(acc, "%{#{key}}", fn _ -> to_string(value) end)
+    end)
+  end
 end