`handle` raises PleaseBackOff when necessary

Fixes #1 (closed)

src/Support.ml

@@ -187,7 +187,9 @@ module Exn = struct
   (* Catching all exceptions. *)
 
   let handle f x =
-    try Ok (f x) with e -> Error e
+    try Ok (f x) with
+    | Engine.PleaseBackOff -> raise Engine.PleaseBackOff
+    | e -> Error e
 
   module Handle = struct