In earlier versions of Go the result was simply "?".
A change in this cycle made the result echo back the hex bytes
of the address, which is certainly useful, but now the result is
not clearly indicating an error. Put the "?" back, at the beginning
of the hex string, to make the invalidity of the string clearer.

Change-Id: I3e0f0b6a005601cd98d982a62288551959185b40
Reviewed-on: https://go-review.googlesource.com/23376
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>