unclear message if sftp unsupported due to paramiko missing

[ThomasWaldmann]

The "Invalid Backend Storage URL" exception msg can be misleading, e.g. if paramiko is not present, sftp: URLs are just unsupported (even for valid, well formed URLs).

See: borgbackup/borg#8574

[ThomasWaldmann]

Improved exception message to include "... or unsupported ...".

The solution for sftp: is usually to use: pip install 'borgstore[sftp]'

Without the [sftp] part, paramiko won't be pulled in as a requirement and without paramiko, borgstore won't have support for sftp:// URLs.

Please note that using sftp protocol is also possible via a rclone:... store using a correctly set up rclone remote for sftp.

[ThomasWaldmann]

It's also already mentioned in the install section of the readme