Notify user if the specified directory does not exist

[andria-dev]

When specifying a directory with the --out=path/to/directory/ flag, if the directory does not exist, none of the icons will be created. While the user is notified that none of the icons were created, they don't know that it's because the directory doesn't exist.

A simple solution would be to display a message saying that the specified directory does not exist and then exiting.

Another option would be to create the directory for them (if possible, if it fails then default to error message and exiting).