Improved error handling.

cluster/test/async/redis/cluster_client.rb

@@ -45,10 +45,11 @@
 	end
 
 	it "can map every slot to a client" do
-		Async::Redis::ClusterClient::HASH_SLOTS.times do |slot|
+		clients = Async::Redis::ClusterClient::HASH_SLOTS.times.map do |slot|
 			client = cluster.client_for(slot)
-
-			expect(client).not.to be_nil
-		end
+		end.uniq
+
+		expect(clients.size).to be == 3
+		expect(clients).not.to have_value(be_nil)
 	end
 end

lib/async/redis/cluster_client.rb

@@ -13,6 +13,9 @@ class ClusterClient
 			class ReloadError < StandardError
 			end
 
+			class SlotError < StandardError
+			end
+
 			Node = Struct.new(:id, :endpoint, :role, :health, :client)
 
 			class RangeMap
@@ -84,9 +87,11 @@ def client_for(slot, role = :master)
 					reload_cluster!
 				end
 
-				nodes = @shards.find(slot)
-
-				nodes = nodes.select{|node| node.role == role}
+				if nodes = @shards.find(slot)
+					nodes = nodes.select{|node| node.role == role}
+				else
+					raise SlotError, "No nodes found for slot #{slot}"
+				end
 
 				if node = nodes.sample
 					return (node.client ||= Client.new(node.endpoint))