Warn when remote_log_conn_id is configured but not found (#67510)

``GCSRemoteLogIO.hook`` silently swallowed ``AirflowNotFoundException``
when the configured ``remote_log_conn_id`` did not exist, falling back
to Application Default Credentials without any signal. That made a
misconfigured remote-log connection (logs effectively going through
the wrong credentials) invisible to operators — a security-control
failure with no operational indication.

Replace the silent ``pass`` with a ``self.log.warning`` mentioning the
unresolved conn_id so the fallback is visible in worker logs. The
behaviour (falling back to ADC) is unchanged for the case where no
``remote_log_conn_id`` is configured.