ceph-crash: reduce log noise from auth fallback in post_crash()

ceph-crash: reduce log noise from auth fallback in post_crash()

post_crash() iterates auth_names trying each identity until one
succeeds. Previously it logged a WARNING for every failed attempt,
even when the next name succeeds. This is noisy in deployments
where only a subset of the auth names have keyrings (e.g. manual
deployments with only client.crash, or environments where
client.admin is not accessible to ceph-crash).

Improve the logging:

- For auth failures (EACCES), log at DEBUG level and continue to
  the next name.
- For non-auth failures, log a WARNING and stop immediately —
  trying other names won't help.
- If all names fail authentication, log a single WARNING with the
  last stderr.

Retain the stderr check from a77b47eeeb57 as a safeguard for mgr
commands that may return rc=0 with an error on stderr.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>