Create host directory when using SyncFolders if it doesn't exist

Fixes #16

Includes error handling, such as:

    $ bundle exec vagrant up --provider=aws
    ...
    [default] Rsyncing folder: /tmp/docker/cant_write/ => ~/docker
    There was an error when attempting to create a shared host folder.
    Please inspect the error message below for more info.
    
    Host path: /tmp/docker/cant_write/
    Error: mkdir: cannot create directory `/tmp/docker': Permission denied

Also fixes a minor typo in the English locale for the rsync error message.