Cluster: Preserve message and suberrors when raising CommandErrorCollection

The constructor signature for CommandErrorCollection is (errors,
message). That means if you call "raise CommandErrorCollection,
'foobar'", that actually winds up setting the _errors_ field to
'foobar', _NOT_ the message. This leads to incredibly undescriptive
"Command errors were replied on any node" errors getting emitted if
there are errors using pipelining.

Fix this by:
1) explicitly constructing the error, with the arguments in the right
order
2) mapping the sub-errors into redis-rb errors too - presumably there's
an :errors attr_reader because somebody will find that useful.

Author: KJ Tsanaktsidis

cluster/lib/redis/cluster/client.rb

@@ -84,7 +84,14 @@ def handle_errors
             raise ERROR_MAPPING.fetch(node_error.class), node_error.message, node_error.backtrace
           end
         end
-        raise ERROR_MAPPING.fetch(error.class), error.message, error.backtrace
+        remapped_node_errors = error.errors.map do |node_key, node_error|
+          remapped = ERROR_MAPPING.fetch(node_error.class, node_error.class).new(node_error.message)
+          remapped.set_backtrace node_error.backtrace
+          [node_key, remapped]
+        end.to_h
+        raise(Redis::Cluster::CommandErrorCollection.new(remapped_node_errors, error.message).tap do |remapped|
+          remapped.set_backtrace error.backtrace
+        end)
       rescue ::RedisClient::Error => error
         raise ERROR_MAPPING.fetch(error.class), error.message, error.backtrace
       end

cluster/test/commands_on_server_test.rb

@@ -21,9 +21,11 @@ def test_bgsave
               'Use BGSAVE SCHEDULE in order to schedule a BGSAVE whenever possible.'
 
     redis_cluster_mock(bgsave: ->(*_) { "-Error #{err_msg}" }) do |redis|
-      assert_raises(Redis::Cluster::CommandErrorCollection, 'Command error replied on any node') do
+      err = assert_raises(Redis::Cluster::CommandErrorCollection, 'Command error replied on any node') do
         redis.bgsave
       end
+      assert_includes err.message, err_msg
+      assert_kind_of Redis::CommandError, err.errors.values.first
     end
   end