Switch to FATAL error for missing checkpoint record without backup_label

Switch to FATAL error for missing checkpoint record without backup_label

Crash recovery started without a backup_label previously crashed with a
PANIC if the checkpoint record could not be found.  This commit lowers
the report generated to be a FATAL instead.

With recovery methods being more imaginative these days, this should
provide more flexibility when handling PostgreSQL recovery processing in
the event of a driver error, similarly to 15f68cebdcec.  An extra
benefit of this change is that it becomes possible to add a test to
check that a FATAL is hit with an expected error message pattern.  With
the recovery code becoming more complicated over the last couple of
years, I suspect that this will be benefitial to cover in the long-term.

The original PANIC behavior has been introduced in the early days of
crash recovery, as of 4d14fe0048cf (PANIC did not exist yet, the code
used STOP).

Author: Nitin Jadhav <nitinjadhavpostgres@gmail.com>
Discussion: https://postgr.es/m/CAMm1aWZbQ-Acp_xAxC7mX9uZZMH8+NpfepY9w=AOxbBVT9E=uA@mail.gmail.com