fix(exports): properly handle export format not being handled

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

.sobelow-skips

@@ -42,3 +42,4 @@ E7967805C1EA5301F2722C7BDB2F25F3
 BDFB0FB1AAF69C18212CBCFD42F8B717
 40220A533CCACB3A1CE9DBF1A8A430A1
 EEB29D1DDA3A3015BC645A989B5BD38E
+5AEE3C678C80E0389C3B0D9D11886EB6

lib/web/controllers/export_controller.ex

@@ -26,10 +26,13 @@ defmodule Mobilizon.Web.ExportController do
           {:error, :not_found}
       end
     else
-      {:error,
-       dgettext("errors", "Export to format %{format} is not enabled on this instance",
-         format: format
-       )}
+      send_resp(
+        conn,
+        404,
+        dgettext("errors", "Export to format %{format} is not enabled on this instance",
+          format: format
+        )
+      )
     end
   end
 end