fix: make an error message easy to understand for the watch command

lib/redis_client/cluster/router.rb

@@ -313,6 +313,8 @@ def send_pubsub_command(method, command, args, &block) # rubocop:disable Metrics
 
       # for redis-rb
       def send_watch_command(command)
+        raise ::RedisClient::Cluster::Transaction::ConsistencyError, 'A block required. And you need to use the block argument as a client for the transaction.' unless block_given?
+
         ::RedisClient::Cluster::OptimisticLocking.new(self).watch(command[1..]) do |c, slot|
           transaction = ::RedisClient::Cluster::Transaction.new(
             self, @command_builder, node: c, slot: slot

test/redis_client/test_cluster.rb

@@ -364,7 +364,7 @@ def test_transaction_in_race_condition
       end
 
       # for redis-rb
-      def test_transaction_with_standalone_watch_command
+      def test_transaction_with_dedicated_watch_command
         @client.call('MSET', '{key}1', '0', '{key}2', '0')
 
         got = @client.call('WATCH', '{key}1', '{key}2') do |tx|
@@ -378,6 +378,12 @@ def test_transaction_with_standalone_watch_command
         assert_equal(%w[1 2], @client.call('MGET', '{key}1', '{key}2'))
       end
 
+      def test_transaction_with_dedicated_watch_command_without_block
+        assert_raises(::RedisClient::Cluster::Transaction::ConsistencyError) do
+          @client.call('WATCH', '{key}1', '{key}2')
+        end
+      end
+
       def test_pubsub_without_subscription
         pubsub = @client.pubsub
         assert_nil(pubsub.next_event(0.01))