SECRETS: Use HTTP error code 504 when a proxy server cannot be reached
Previously, a generic 500 error code was returned. This patch adds a new
error message on a failure to contact the proxy server and returns 504,
"Gateway timeout" instead.
Resolves:
https://fedorahosted.org/sssd/ticket/3212
Reviewed-by: Fabiano FidĂȘncio <fidencio@redhat.com>