Do we want to log this for all errors?  (Including Cassandra timeout errors?)  Because that error would pass through the `lock_not_acquired` errback straight to this errback.  If so, do we want to log the type of error?  (e.g. `"Could not acquire lock in {} seconds due to {} error"`.format(seconds, error_type)` or some such?

If we only want to log this if the failure is because of BusyLockError or some such, maybe we should do `if self._log and failure.check(BusyLockError):`?