Improve Backend Error page

Mostly handle things properly when the front-end is missing

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/web/views/auth_view.ex

@@ -23,7 +23,12 @@ defmodule Mobilizon.Web.AuthView do
       Tag.tag(:meta, name: "auth-user-actor-id", content: user_actor_id)
     ]
 
-    tags = Instance.build_tags() ++ info_tags
-    inject_tags(tags, get_locale(conn))
+    with tags <- Instance.build_tags() ++ info_tags,
+         {:ok, html} <- inject_tags(tags, get_locale(conn)) do
+      html
+    else
+      {:error, error} ->
+        return_error(conn, error)
+    end
   end
 end