Suppress connection errors when backup slurmdbd is active

When the backup slurmdbd is active, there is no need to print
connection errors every 10 minutes when the backup checks in on the
primary, at least at the 'error' level.

Prevent dbd clients, like sacctmgr, from printing a connection error
when running commands against the backup slurmdbd unless debug2.
Before, the clients would always print errors when trying to access
the primary dbd first.

Bug 5823