Problem:
Currently we support only http and https links. If there is an `ftp://`
link, you will get exception.
Solution:
Use `ftp-client` to check connection to ftp, see response statuses and
check file existence. This produces adding new error types and small
refactoring.
Provide a test which is separate executable, where we have to pass CLA -
ftp host.
Co-authored-by: Alexander Bantyev <alexander.bantyev@serokell.io>
