Catch general "OperationalError: database is locked" in worker validation instead of throwing BadMachine

[AndyGrant]

Occurs in volume SPSA assignments, in particular with lightweight databases like sqlite3 as packaged by default in django. Seems to not occur in MySQL, as it was never witnessed in high concurrency SPSA sessions on Torch Bench. However, its unclear if one would actually "witness" any problems, if not looking at the client output, aside from an increase in volume of new machine registrations.