commands/rdp: error if RDP info can't be detected

plugins/commands/rdp/command.rb

@@ -30,9 +30,12 @@ module VagrantPlugins
               name: machine.guest.name.to_s
           end
 
-          @env.host.capability(
+          rdp_info = machine.guest.capability(:rdp_info)
-            :rdp_client,
+          if !rdp_info
-            machine.guest.capability(:rdp_info))
+            raise Errors::RDPUndetected
+          end
+
+          @env.host.capability(:rdp_client, rdp_info)
         end
       end
     end

plugins/commands/rdp/errors.rb

@@ -13,6 +13,10 @@ module VagrantPlugins
       class HostUnsupported < RDPError
         error_key(:host_unsupported)
       end
+
+      class RDPUndetected < RDPError
+        error_key(:rdp_undetected)
+      end
     end
   end
 end

templates/locales/command_rdp.yml

@@ -19,3 +19,8 @@ en:
         an RDP client, please contribute this functionality back
         into Vagrant. At the very least, open an issue on how it
         could be done and we can handle the integration.
+      rdp_undetected: |-
+        RDP connection information for this machine could not be
+        detected. This is typically caused when we can't find the IP
+        or port to connect to for RDP. Please verify you're forwarding
+        an RDP port and that your machine is accessible.