Use http error code 422 in CheckView when a merge is not possible. Previously the response used the code 500 in this case. This is not useful because it's not an internal server error when the check whether a PDF file can be merged fails because that's the point of the CheckView. The code used now means "Unprocessable Entity" and fits better. Part of mpuls/issue6009

# -*- coding: utf-8 -*-
if __name__ == "__main__":
    from odfcast import app

    server_name = app.config['SERVER_NAME']
    if server_name and ':' in server_name:
        host, port = server_name.rsplit(':', 1)
        port = int(port)
    else:
        port = 5000
        host = "127.0.0.1"

    app.run(host=host, port=port)