fix: graceful shutdown with asynchronous processes - fixes #1022

pkg/gotenberg/shutdown.go

@@ -0,0 +1,8 @@
+package gotenberg
+
+import "errors"
+
+// ErrCancelGracefulShutdownContext tells that a module wants to abort a
+// graceful shutdown and stops Gotenberg right away as there are no more
+// ongoing processes.
+var ErrCancelGracefulShutdownContext = errors.New("cancel graceful shutdown's context")