Attach connection errors to socket

Raising the errors would crash the current thread. As this thread
contains the Eventmachine, that would influence other connections
as well. Attaching the errors to the connection and reading them
after the connection was closed ensures that the thread stays
alive while handling the errors in the main thread of the current
request.

lib/runner/strategy/docker_container_pool.rb

@@ -68,6 +68,7 @@ class Runner::Strategy::DockerContainerPool < Runner::Strategy
       socket.close(:timeout)
       destroy_at_management
     end
+    socket
   end
 
   private