rebalancer: add replicaset.id into rebalancer_request_state errors

Before this patch the function `rebalancer_download_states` didn't
return information about replicaset from which the states could not
be downloaded. As a result, the errors returned from
`rebalancer_download_states` lack of valuable information about
unhealthy replicaset.

Now, we add replicaset.id into error which is returned from the
`rebalancer_download_states`. It can help us to propagate replicaset.id
to rebalancer service.

Closes #212

NO_DOC=bugfix

Author: mrForza

test/storage-luatest/storage_1_1_1_test.lua

@@ -227,3 +227,21 @@ test_group.test_rebalancer_routes_logging = function(g)
         g.replica_1_a:grep_log('The cluster is balanced ok.')
     end)
 end
+
+--
+-- Add replicaset.id into rebalancer_request_state errors (gh-212).
+--
+test_group.test_no_log_spam_when_buckets_no_active = function(g)
+    local moved_bucket = vtest.storage_first_bucket(g.replica_1_a)
+    move_bucket(g.replica_1_a, g.replica_2_a, moved_bucket)
+    vtest.storage_stop(g.replica_2_a)
+    local err_log = string.format('Error during downloading rebalancer ' ..
+                                  'states:.*"replicaset_id":"%s"',
+                                  g.replica_2_a:replicaset_uuid())
+    t.helpers.retrying({timeout = 60}, function()
+        g.replica_1_a:exec(function() ivshard.storage.rebalancer_wakeup() end)
+        t.assert(g.replica_1_a:grep_log(err_log))
+    end)
+    vtest.storage_start(g.replica_2_a, global_cfg)
+    move_bucket(g.replica_2_a, g.replica_1_a, moved_bucket)
+end

vshard/storage/init.lua

@@ -2812,6 +2812,7 @@ local function rebalancer_download_states()
             replicaset, 'vshard.storage.rebalancer_request_state', {},
             {timeout = consts.REBALANCER_GET_STATE_TIMEOUT})
         if state == nil then
+            err.replicaset_id = replicaset.id
             return nil, err
         end
         local bucket_count = state.bucket_active_count +