Raise proper error if there aren't any NIC slots available

lib/vagrant/errors.rb

@@ -437,6 +437,10 @@ module Vagrant
       error_key(:virtualbox_invalid_version)
     end
 
+    class VirtualBoxNoRoomForHighLevelNetwork < VagrantError
+      error_key(:virtualbox_no_room_for_high_level_network)
+    end
+
     class VirtualBoxNotDetected < VagrantError
       status_code(8)
       error_key(:virtualbox_not_detected)

plugins/providers/virtualbox/action/network.rb

@@ -51,7 +51,7 @@ module VagrantPlugins
             slot = options[:adapter]
             if !slot
               if available_slots.empty?
-                # TODO: Error that we have no room for this adapter
+                raise Vagrant::Errors::VirtualBoxNoRoomForHighLevelNetwork
               end
 
               slot = available_slots.shift

templates/locales/en.yml

@@ -280,6 +280,13 @@ en:
         VirtualBox is complaining that the installation is incomplete. Please
         run `VBoxManage --version` to see the error message which should contain
         instructions on how to fix this error.
+      virtualbox_no_room_for_high_level_network: |-
+        There is no available slots on the VirtualBox VM for the configured
+        high-level network interfaces. "private_network" and "public_network"
+        network configurations consume a single network adapter slot on the
+        VirtualBox VM. VirtualBox limits the number of slots to 8, and it
+        appears that every slot is in use. Please lower the number of used
+        network adapters.
       virtualbox_not_detected: |-
         Vagrant could not detect VirtualBox! Make sure VirtualBox is properly installed.
         Vagrant uses the `VBoxManage` binary that ships with VirtualBox, and requires