Use fail_with

modules/exploits/multi/misc/java_rmi_server.rb

@@ -112,13 +112,11 @@ class Metasploit3 < Msf::Exploit::Remote
     end
 
     if buf =~ /RMI class loader disabled/
-      print_error("Not exploitable: the RMI class loader is disabled")
-      return
+      fail_with(Failure::NotVulnerable, "The RMI class loader is disabled")
     end
 
     if buf =~ /java.lang.ClassNotFoundException/
-      print_error("Not exploitable: the RMI class loader doesn't find the payload")
-      return
+      fail_with(Failure::Unknown, "The RMI class loader couldn't find the payload")
     end
 
     print_good("Target #{rhost}:#{rport} may be exploitable...")