Print message when host name can't be found

If the assined IP doesn't have an alias like 1-2-3-4.example.com the script would fail silently

This commit print out a message telling what happen

examples/main.tf

@@ -3,5 +3,5 @@ module "website_url" {
   jackdaw_username = "user"
   jackdaw_key      = "key"
   alias            = "alias.hpc.cam.ac.uk"
-  ip               = "17.1.2.3"
+  ip               = "128.232.226.16"
 }