OpenTSDB: Fixed a bug that a thread hung when HBase scanner failed. (https://rbcommons.com/s/pepperdata/r/3864)
- The bug was caused by the missing callback that catches an exception that
HBase scanner threw when it failed to retry. So the exception was not passed to
the thread that is waiting for the HBase scanner and the thread waited forever.
- The fix was adding errback to HBase scanner.