Add information about lock file path

lib/vagrant/action/builtin/box_add.rb

@@ -485,7 +485,8 @@ module Vagrant
           mutex_path = d.destination + MUTEX_SUFFIX
           if File.file?(mutex_path)
             raise Errors::DownloadAlreadyInProgress,
-              dest_path: d.destination
+              dest_path: d.destination,
+              lock_file_path: mutex_path
           end
 
           begin

templates/locales/en.yml

@@ -901,6 +901,7 @@ en:
         a file to the same location.
 
         Download path: %{dest_path}
+        Lock file path: %{lock_file_path}
       downloader_error: |-
         An error occurred while downloading the remote file. The error
         message, if any, is reproduced below. Please fix this error and try