This reverts commit 71cd9e1.
The goroutine that attaches to stdin should send an error in the channel only if copy is called, otherwise hijack may return just because of a nil error returned by the stdin goroutine. This doesn't affect the behavior of all the background copying, but swallows errors in the stdout goroutine. Here's a build failure caused by this bug: https://travis-ci.org/tsuru/tsuru/jobs/59943962#L3285. Something changed at go tip and exposed this bug.