Add error message for when out/active is not set

Without this, run-cts fails with no indiciation on how to fix
things.

Change-Id: I1a7839fce5c8f578079574e58111747db3f93ee3
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/208934
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Commit-Queue: Gregg Tavares <gman@chromium.org>
diff --git a/tools/src/cmd/run-cts/node/cmd.go b/tools/src/cmd/run-cts/node/cmd.go
index d1865c3..4b00237 100644
--- a/tools/src/cmd/run-cts/node/cmd.go
+++ b/tools/src/cmd/run-cts/node/cmd.go
@@ -40,7 +40,6 @@
 	"dawn.googlesource.com/dawn/tools/src/cov"
 	"dawn.googlesource.com/dawn/tools/src/dawn/node"
 	"dawn.googlesource.com/dawn/tools/src/fileutils"
-	"dawn.googlesource.com/dawn/tools/src/subcmd"
 )
 
 type flags struct {
@@ -171,7 +170,7 @@
 func (c *cmd) processFlags() error {
 	// Check mandatory arguments
 	if c.flags.bin == "" {
-		return subcmd.InvalidCLA()
+		return fmt.Errorf("-bin is not set. It defaults to <dawn>/out/active (%v) which does not exist", filepath.Join(fileutils.DawnRoot(), "out/active"))
 	}
 	if !fileutils.IsDir(c.flags.bin) {
 		return fmt.Errorf("'%v' is not a directory", c.flags.bin)