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 <mdlayher@gmail.com>
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/cmd/coordinator/modproxy.go b/cmd/coordinator/modproxy.go
index 077df12..9df8e24 100644
--- a/cmd/coordinator/modproxy.go
+++ b/cmd/coordinator/modproxy.go
@@ -42,8 +42,8 @@
 		return
 	}
 	if !strings.Contains(builder, "-") || builderKey(builder) != pass {
-		http.Error(w, "bad username or password", http.StatusUnauthorized)
 		log.Printf("modproxy: sending 401 Unauthorized due to invalid key for builder %q", builder)
+		http.Error(w, "bad username or password", http.StatusUnauthorized)
 		return
 	}