internal/poll: return error if WriteConsole fails

Fixes #19068.

Change-Id: Id76037826376b5fe8b588fe3dc02182dfaff8c21
Reviewed-on: https://go-review.googlesource.com/36935
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/internal/poll/fd_windows.go

@@ -624,7 +624,7 @@ func (fd *FD) writeConsole(b []byte) (n int, err error) {
 			var written uint32
 			err = syscall.WriteConsole(fd.Sysfd, &uint16s[0], uint32(len(uint16s)), &written, nil)
 			if err != nil {
-				return 0, nil
+				return 0, err
 			}
 			uint16s = uint16s[written:]
 		}