Rebalance fails when adding back a failed over node with delta recovery

Build : 4.6.3-4059

This issue is seen often in a system test which scales upto 100M+ items. There is a step to fail over a KV node and then add it back to the cluster with delta recovery. This rebalance sometimes fails.

[user:info,2017-07-13T08:18:48.237Z,ns_1@172.31.2.128:<0.980.0>:ns_orchestrator:handle_info:525]Rebalance exited with reason

[error_logger:error,2017-07-13T08:18:48.237Z,ns_1@172.31.2.128:error_logger<0.6.0>:ale_error_logger_handler:do_log:203]
=========================CRASH REPORT=========================
crasher:
initial call: erlang:apply/2
pid: <0.31843.10>
registered_name: []
exception error: no match of right hand side value fail
in function ns_rebalancer:'apply_delta_recovery_buckets/3-fun-0'/2 (src/ns_rebalancer.erl, line 1122)
in call from lists:foreach/2 (lists.erl, line 1323)
in call from ns_rebalancer:apply_delta_recovery_buckets/3 (src/ns_rebalancer.erl, line 1120)
in call from ns_rebalancer:rebalance/6 (src/ns_rebalancer.erl, line 598)
ancestors: [<0.980.0>,ns_orchestrator_sup,mb_master_sup,mb_master,
<0.812.0>,ns_server_sup,ns_server_nodes_sup,<0.166.0>,
ns_server_cluster_sup,<0.89.0>]
messages: []
links: [<0.980.0>]
dictionary: []
trap_exit: false
status: running
heap_size: 75113
stack_size: 27
reductions: 376473
neighbours: