ssh: clarify error type if a SendRequest goes unanswered This came up in the discussion of issue #16194. Change-Id: Ibb4cadf3be975377819eca2e41e0992794ada77b Reviewed-on: https://go-review.googlesource.com/24820 Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org> Run-TryBot: Han-Wen Nienhuys <hanwen@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org>