Throw retriable exception for getBlockLocations when ObserverNameNode is in safemode

    try {
      checkOperation(OperationCategory.READ);
      res = FSDirStatAndListingOp.getBlockLocations(
          dir, pc, srcArg, offset, length, true);
      if (isInSafeMode()) {
        for (LocatedBlock b : res.blocks.getLocatedBlocks()) {
          // if safemode & no block locations yet then throw safemodeException
          if ((b.getLocations() == null) || (b.getLocations().length == 0)) {
            SafeModeException se = newSafemodeException(
                "Zero blocklocations for " + srcArg);
            if (haEnabled && haContext != null &&
                haContext.getState().getServiceState() == HAServiceState.ACTIVE) {
              throw new RetriableException(se);
            } else {
              throw se;
            }
          }
        }
      }