`sardine-web` not installed: hint users towards possible install

[Bubobubobubobubo]

sardine-web is an optional package for Sardine that implements a text editor. The editor can be launched using the sardine web command. When the package is not installed, the message pasted above is returned. No message is hinting the user towards a possible install.

Try 'sardine -h' for help.

Error: No such command 'web'.

TODO: implement a message providing instructions for installation.