Revert^4 "allow Ninja variables in RuleBuilder API"

789a7e0883d5e142ff98898ac02797f106d38b9c

Change-Id: Iee1156adf0ad6e6d2ae7e22d90a447d4b564e59f
diff --git a/cmd/sbox/sbox.go b/cmd/sbox/sbox.go
index fc56dd5..3364f50 100644
--- a/cmd/sbox/sbox.go
+++ b/cmd/sbox/sbox.go
@@ -119,6 +119,9 @@
 	}
 
 	manifest, err := readManifest(manifestFile)
+	if err != nil {
+		return err
+	}
 
 	if len(manifest.Commands) == 0 {
 		return fmt.Errorf("at least one commands entry is required in %q", manifestFile)