Now that the build system has been updated to install x/tools in
$GOPATH (CL 149658), depend on it being there and don't ignore
failures to build the tool.

Update to CL 149097.

Change-Id: I72fde347217533697068b6a6773696cc2b83e9ed
Reviewed-on: https://go-review.googlesource.com/c/150017
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>