Introduce support for custom nginx error pages

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

lib/service/error_page.ex

@@ -0,0 +1,17 @@
+defmodule Mobilizon.Service.ErrorPage do
+  @moduledoc """
+  Render an error page
+  """
+
+  def init do
+    render_error_page()
+  end
+
+  defp render_error_page do
+    content =
+      Phoenix.View.render_to_string(Mobilizon.Web.ErrorView, "500.html", conn: %Plug.Conn{})
+
+    path = Path.join(Application.app_dir(:mobilizon, "priv/errors"), "error.html")
+    File.write(path, content)
+  end
+end