Use a more correct error message

-1 is a valid session ID, even though it's a fake one.

lib/msf/base/simple/post.rb

@@ -93,19 +93,18 @@ protected
   #
   def self.job_run_proc(ctx)
     mod = ctx[0]
-    sid = mod.datastore["SESSION"]
     begin
       mod.setup
       mod.framework.events.on_module_run(mod)
       # Grab the session object since we need to fire an event for not
       # only the normal module_run event that all module types have to
       # report, but a specific event for sessions as well.
-      s = mod.framework.sessions.get(sid)
+      s = mod.framework.sessions.get(mod.datastore["SESSION"])
       if s
         mod.framework.events.on_session_module_run(s, mod)
         mod.run
       else
-        mod.print_error("Invalid session ID: #{sid}")
+        mod.print_error("Session not found")
         mod.cleanup
         return
       end