As discussed in #18095 the server should not log for bad user input.

Change-Id: I628a796926eff3a971e5b04abec17ea377c3f9b7
Reviewed-on: https://go-review.googlesource.com/33617Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>