Suppress stderr ra.get_certificate() failures

[rcritten]

When the subsystemCert certificate is invalid healthcheck will spew 10 or so messages like this on stderr:

ra.get_certificate(): Request failed with status 403: Non-2xx response from CA REST API: 403. (403)

This should be suppressed. A similar message appears in the healthcheck report:

{
    "source": "ipahealthcheck.ipa.certs",
    "check": "IPACertRevocation",
    "result": "ERROR",
    "uuid": "65765f34-c757-432c-b6e3-0d4fcba18516",
    "when": "20240123183745Z",
    "duration": "73.755750",
    "kw": {
      "key": "20210522171136",
      "serial": 268304549,
      "error": "Certificate operation cannot be completed: Request failed with status 403: Non-2xx response from CA REST API: 403.  (403)",
      "msg": "Request for certificate serial number {serial} in request {key} failed: {error}"
    }
  },