Do not fail_with on method used from check

modules/exploits/unix/webapp/zeroshell_exec.rb

@@ -99,7 +99,7 @@ class Metasploit3 < Msf::Exploit::Remote
       print_status("#{peer} - Password retrieved [ #{pass} ]")
       return pass
     else
-      fail_with(Failure::Unknown, "#{peer} - Retrieving password failed!")
+      return nil
     end
   end
 
@@ -130,6 +130,10 @@ class Metasploit3 < Msf::Exploit::Remote
   def execute_command(cmd, opts)
 
     admin_password = password
+    if admin_password.nil?
+      fail_with(Failure::Unknown, "#{peer} - Retrieving password failed!")
+    end
+
     session = login(admin_password)
 
     scriptname = rand_text_alphanumeric(8)