webadmin : gluster volume rebalance status error handling

Errors like,

1. If some/all the hosts forming a distribute volume with atleast 2 bricks are

   down, and if rebalance status is triggered on such a volume, approprite

   error message is displayed.

2. If any n/w related or any other exceptions are thrown,they are also

   handled properly.

And hence the popup doesn't hang anymore.

Change-Id: Ie13748716f23a797fc0e3ec16acbb4150c43b7e8
Signed-off-by: Anmol Babu <anbabu@redhat.com>