Display warning message if Vivio Swift can't write to it's directory
There's nothing in the logs when this happens, so it needs to be displayed.
Example situation: WP installed and running in a directory as one user. WP-CLI comes in as root and installs the plugin. This will install the plugin with root privs, which the site can't write to.
To upload designs, you'll need to enable LFS and have admin enable hashed storage. More information