warn on method failure instead of error

Author: busterb

modules/exploits/linux/misc/drb_remote_codeexec.rb

@@ -117,7 +117,7 @@ class << p
         handler(nil)
         break
       rescue SecurityError => e
-        print_error("target is not vulnerable to #{method}")
+        print_warning("target is not vulnerable to #{method}")
       end
     end