Salvages substantive part of #26490 by @aqilaziz. Detects corrupt board
DBs ("file is not a database" / "database disk image is malformed")
and disables them by fingerprint until they're repaired, instead of
flooding the gateway log with repeated logger.exception tracebacks every
tick.
Cherry-picked the substantive commit (ea5b4ec2a); the tip commit was
an unrelated _is_dir OSError fix for service-path lookup. Dropped a
small test reformat that was bundled in the same commit.
d37574775b