Change failure code

Author: jvazquez-r7

modules/exploits/linux/http/vap2500_tools_command_exec.rb

@@ -69,7 +69,7 @@ def exploit
     print_status("#{peer} - Trying to access the device ...")
 
     unless check == Exploit::CheckCode::Vulnerable
-      fail_with(Failure::Unknown, "#{peer} - Failed to access the vulnerable device")
+      fail_with(Failure::NotVulnerable, "#{peer} - Failed to access the vulnerable device")
     end
 
     print_status("#{peer} - Exploiting...")