sftpd.py: switch from using bare asserts to assertutil.

Signed-off-by: David-Sarah Hopwood <david-sarah@jacaranda.org>
Improve SFTP error handling and remove use of IFinishableConsumer. fixes #1525

Signed-off-by: David-Sarah Hopwood <david-sarah@jacaranda.org>
New Welcome page: ensure 'not connected' status for Introducer and Helper is shown via text as well as bullet colour. refs #1713

Signed-off-by: David-Sarah Hopwood <david-sarah@jacaranda.org>