404 if eventdownload file does not exist

[ghost]

If the file for the organization is absent, a 502 Bad Gateway is shown. Instead an error message that there is no file shown. E.g.

Keine Daten gefunden. Die Dateien werden einmal täglich erstellt.

As files are generated only once per day, this can happen if someone adds a handle and then tries to download.

[MarkHofstetter]

I see a 404, ie no 502
{
"message": "Resource not found",
"status": "not found"
}

which should be handled by the frontend