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>
1 file changed