Add response check

Author: Takahiro-Yoko

modules/exploits/linux/http/empire_skywalker.rb

@@ -286,6 +286,7 @@ def exploit
         'uri'       => normalize_uri(target_uri.path, datastore['STAGE_PATH']),
         'headers' => {'user-agent' => datastore['AGENT']}
       })
+      fail_with(Failure::Unknown, "Failed to communicate with STAGE2") unless res and res.code == 200
       aes_decrypt(session_key, res.body)
 
       log_path = "/var/lib/powershell-empire/empire/server/downloads/#{session_id}/agent.log"