cephadm: have agent check for errors before json loading mgr response

[adk3798]

Currently, since it tries to json.loads the response payload before checking the return code, if there was an error it fails with

Failed to send metadata to mgr: the JSON object must be str, bytes or bytearray, not ConnectionRefusedError

which is masking the actual failure.

Fixes: https://tracker.ceph.com/issues/65553

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[adk3798]

https://pulpito.ceph.com/adking-2024-04-30_05:42:49-orch:cephadm-wip-adk-testing-2024-04-29-2009-distro-default-smithi/

Most failures were in cluster log failures that are still in the process of being cleaned up.

Besides that, failures were:

• 1 instance of https://tracker.ceph.com/issues/65718 (known issue)
• 4 instances of mds_upgrade_sequence test failing (known issue)
• 1 instance of staggered upgrade with agent failing (known issue)
• some dead jobs, either reimaging issue or getting stuck before the test has actually began (seemingly on nvme-loop task) (known issue)