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/page_view.ex

@@ -63,16 +63,26 @@ defmodule Mobilizon.Web.PageView do
 
   def render(page, %{object: object, conn: conn} = _assigns)
       when page in ["actor.html", "event.html", "comment.html", "post.html"] do
-    locale = get_locale(conn)
-    tags = object |> Metadata.build_tags(locale)
-    inject_tags(tags, locale)
+    with locale <- get_locale(conn),
+         tags <- object |> Metadata.build_tags(locale),
+         {:ok, html} <- inject_tags(tags, locale) do
+      html
+    else
+      {:error, error} ->
+        return_error(conn, error)
+    end
   end
 
   # Discussions are private, no need to embed metadata
   def render("discussion.html", params), do: render("index.html", params)
 
   def render("index.html", %{conn: conn}) do
-    tags = Instance.build_tags()
-    inject_tags(tags, get_locale(conn))
+    with tags <- Instance.build_tags(),
+         {:ok, html} <- inject_tags(tags, get_locale(conn)) do
+      html
+    else
+      {:error, error} ->
+        return_error(conn, error)
+    end
   end
 end