Remove obsolete reference to FTP
Remove obsolete reference to FTP

FTP support is being removed from Chrome (see
https://www.ghacks.net/2019/08/16/google-chrome-82-wont-support-ftp-anymore/)

Also re-word the error message slightly to be clearer about the protocol
restriction.

hypothesis/browser-extension,hypothesis/browser-extension,hypothesis/browser-extension,hypothesis/browser-extension