SERVER-9481 Fixes fluctuating between ROLLBACK and SECONDARY when ROLLBACK is impossible

Only leave ROLLBACK when successful and go to FATAL when ROLLBACK is impossible
because we are not willing to rollback so much data

Signed-off-by: Matt Dannenberg <matt.dannenberg@10gen.com>
SERVER-9481 further rollback exception cleanup
SERVER-7728 Add more details on why a shard is 'unavailable' for balancing