[tools][perfmon]: Post error on `gclient sync` failure.

Prevents endless retries on the same CL, if DEPS are bad.

Change-Id: Ibdbe9517c8eafa84d5c87f9f05a516f81103a386
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/146640
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
diff --git a/tools/src/cmd/perfmon/main.go b/tools/src/cmd/perfmon/main.go
index bcaf8e9..77f4aa1 100644
--- a/tools/src/cmd/perfmon/main.go
+++ b/tools/src/cmd/perfmon/main.go
@@ -716,7 +716,7 @@
 		"sync",
 		"--force",
 	); err != nil {
-		return fmt.Errorf("failed to fetch dawn dependencies:\n  %w", err)
+		return errFailedToBuild{reason: fmt.Errorf("failed to fetch dawn dependencies:\n  %w", err)}
 	}
 	return nil
 }