Do better check method

modules/exploits/multi/http/cisco_dcnm_upload.rb

@@ -86,14 +86,18 @@ class Metasploit3 < Msf::Exploit::Remote
       'method' => 'GET'
     })
 
-    if res and
-        res.code == 200 and
+    unless res
+      return Exploit::CheckCode::Unknown
+    end
+
+    if res.code == 200 and
         res.body.to_s =~ /Data Center Network Manager/ and
         res.body.to_s =~ /<div class="productVersion">Version: (.*)<\/div>/
       version = $1
       print_status("Cisco Primer Data Center Network Manager version #{version} found")
-    else
-      return Exploit::CheckCode::Safe
+    elsif res.code == 200 and
+        res.body.to_s =~ /Data Center Network Manager/
+      return Exploit::CheckCode::Detected
     end
 
     if version =~ /6.1/