cmd/coordinator: move log.Printf before http.Error for consistency with nearby code
cmd/coordinator: move log.Printf before http.Error for consistency with nearby code

From CL 170620 feedback.

Change-Id: I4434a04d46bc53be81133c860835a8944677db52
Reviewed-on: https://go-review.googlesource.com/c/build/+/170622
Reviewed-by: Matt Layher <76528ff2e917253aa2211f68e4ca60b5b846dcf5@gmail.com>
Run-TryBot: Matt Layher <76528ff2e917253aa2211f68e4ca60b5b846dcf5@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/build,golang/build,golang/build,golang/build