test: fix an error handling bug

[supercaracal]

TestAgainstClusterScale#test_02_scale_in:
RedisClient::CommandError: ERR Please use SETSLOT only with masters. (redis://127.0.0.1:6385)
    vendor/bundle/ruby/3.3.0/gems/redis-client-0.21.0/lib/redis_client/connection_mixin.rb:36:in `call'
    vendor/bundle/ruby/3.3.0/gems/redis-client-0.21.0/lib/redis_client.rb:279:in `block (2 levels) in call'
    vendor/bundle/ruby/3.3.0/gems/redis-client-0.21.0/lib/redis_client/middlewares.rb:16:in `call'
    vendor/bundle/ruby/3.3.0/gems/redis-client-0.21.0/lib/redis_client.rb:278:in `block in call'
    vendor/bundle/ruby/3.3.0/gems/redis-client-0.21.0/lib/redis_client.rb:699:in `ensure_connected'
    vendor/bundle/ruby/3.3.0/gems/redis-client-0.21.0/lib/redis_client.rb:277:in `call'
    test/cluster_controller.rb:147:in `block in finish_resharding'

Although it's a bad way of the error handling.