When failing to fetch the target account, the ProcessingWorker fails
as expected, but since it hasn't cleared the `move_in_progress` flag,
the next attempt at processing skips the `Move` activity altogether.
This commit changes it to clear the flag when encountering any
unexpected error on fetching the target account. This is likely to
occur because, of, e.g., a timeout, when many instances query the
same actor at the same time.
