Previously, write-back errors from Complete/DeadLetter/FailWithRetry
were silently dropped via underscore assignment. A transient DB
failure here would leave a job stuck (still 'running' with active
lease) until the reaper resets it, with zero observability into why.
Log each write-back error at Error level so operators can correlate
stuck jobs with DB symptoms. Code quality review of T8.
q792602257