Return an exception struct from put_locale_from_session/2 for an empty session

lib/localize/plug.ex

@@ -8,6 +8,14 @@ defmodule Localize.Plug do
 
   @session_key Localize.Plug.PutLocale.session_key()
 
+  defmodule NoSessionLocaleError do
+    @moduledoc """
+    Returned by `Localize.Plug.put_locale_from_session/2` when the session has no locale.
+    """
+
+    defexception message: "No locale was found in the session"
+  end
+
   @doc """
   Puts the locale from the session into the current process.
 
@@ -29,7 +37,8 @@ defmodule Localize.Plug do
 
   * `{:ok, locale}` or
 
-  * `{:error, {exception, reason}}`
+  * `{:error, exception}`. A session without a locale returns a
+    `Localize.Plug.NoSessionLocaleError`.
 
   ### Examples
 
@@ -61,7 +70,7 @@ defmodule Localize.Plug do
   end
 
   def put_locale_from_session(_session, _options) do
-    {:error, {Localize.UnknownLocaleError, "No locale was found in the session"}}
+    {:error, %NoSessionLocaleError{}}
   end
 
   # A locale that Gettext has no translations for is a configuration

mix.exs

@@ -76,6 +76,7 @@ defmodule LocalizeWeb.MixProject do
       groups_for_modules: [
         Plugs: [
           Localize.Plug,
+          Localize.Plug.NoSessionLocaleError,
           Localize.Plug.PutLocale,
           Localize.Plug.PutSession,
           Localize.Plug.AcceptLanguage,

test/localize_web_test.exs

@@ -51,8 +51,11 @@ defmodule LocalizeWebTest do
     assert Localize.Plug.PutLocale.locale_from_host(nil) == nil
   end
 
-  test "put_locale_from_session returns error when no locale in session" do
-    assert {:error, _} = Localize.Plug.put_locale_from_session(%{})
+  test "put_locale_from_session returns a NoSessionLocaleError for an empty session" do
+    assert {:error, %Localize.Plug.NoSessionLocaleError{} = error} =
+             Localize.Plug.put_locale_from_session(%{})
+
+    assert Exception.message(error) == "No locale was found in the session"
   end
 
   test "Phoenix.HTML.Safe protocol for Localize.LanguageTag" do