pwclient should report more user-friendly message on Connection Refused

[stephenfin]

From an email exchange, pwclient failed with:

[long traceback snipped]
socket.error: [Errno 111] Connection refused

This looks like what would happen if you tried to use pwclient when our
web server was down. This happens every now and then for very short
periods of time when we do updates to various things (including
libraries that the web server itself depends on).

Do the developers: maybe it is worth catching this error in particular
and giving some reasonable message (or, optionally,trying again after a
short wait - or both).

Do this in the pwclient rest api rewrite.

This was originally filed against Patchwork.