Fix error propagation from userland-proxy

I've taken care about this issue(original PR: #7927), because @erikh is pretty busy with linksv2.

Fixes #7935
@crosbymichael I hope it's okay, I cut a branch from this and followed up your comments with a commit.

https://github.com/LK4D4/docker/pull/5

cc/ @LK4D4