Download root enhancements

This submission contains some cleanup work for the PE install archive. It validates the download_root config option and has more useful error text of failures. Now when the installer is not available, the use has some information to work off of at least. The error text now lets them know what versions are available by calling essentially `vagrant pe-build list` and directing the user to either provide a download_root source or use `vagrant pe-build` to manually provide an installer. I left where to get the installer as a more implicit step for users. We could always add a line directing them to the PL download page later on.

Along the way, I had add the I18n support, which should just wind up being useful in the future anyway.