Normalize the return value from reply.

Resolves:

riak_kv_buckets_fsm.erl:80: Expression produces a value of type 'ok' | {non_neg_integer(),'done' | {'buckets',[any()]} | {'buckets_stream',[any()]} | {'error',_}}, but this value is unmatched
riak_kv_buckets_fsm.erl:98: Expression produces a value of type {non_neg_integer(),'done' | {'buckets',[any()]} | {'buckets_stream',[any()]} | {'error',_}}, but this value is unmatched
riak_kv_buckets_fsm.erl:102: Expression produces a value of type {non_neg_integer(),'done' | {'buckets',[any()]} | {'buckets_stream',[any()]} | {'error',_}}, but this value is unmatched
riak_kv_buckets_fsm.erl:108: Expression produces a value of type {non_neg_integer(),'done' | {'buckets',[any()]} | {'buckets_stream',[any()]} | {'error',_}}, but this value is unmatched