meta: exit generator with an error when invoked wrong

go-generate.go

@@ -1,11 +1,12 @@
 package main
 
-import "fmt"
+import "os"
 
 //go:generate go run ./horsegen
 //go:generate go generate ./go/...
 //go:generate go fmt ./...
 
 func main() {
-	fmt.Println("go generate, not go run")
+	os.Stderr.WriteString("go generate, not go run\n")
+	os.Exit(2)
 }