Add proper message when Vagrant is quit from interrupt

lib/vagrant/errors.rb

@@ -206,6 +206,7 @@ module Vagrant
 
     class VagrantInterrupt < VagrantError
       status_code(40)
+      error_key(:interrupted)
     end
 
     class VirtualBoxInvalidOSE < VagrantError

templates/locales/en.yml

@@ -8,6 +8,7 @@ en:
       base_vm_not_found: The base VM with the name '%{name}' was not found.
       box_not_found: Box '%{name}' could not be found.
       cli_missing_env: This command requires that a Vagrant environment be properly passed in as the last parameter.
+      interrupted: Vagrant exited after cleanup due to external interrupt.
       multi_vm_required: A multi-vm environment is required for name specification to this command.
       multi_vm_target_required: `vagrant %{command}` requires a specific VM name to target in a multi-VM environment.
       no_env: No Vagrant environment detected. Run `vagrant init` to set one up.