Do not ask for password if user does not exist
When a password is not in the password file, or the login
failed for an unknown reason, check the user exists using
API allusers before halting to request the user enter a
password for a user which may not exist.
This allows a graceful error when an invalid username
is configured for a site, instead of halting for user
input which may not be possible if automated process,
and will be useless anyway as the user does not exist.
This may occur if a username is set for all sites in a
family, if the user does not exist on all sites, which
can be a problem the user cant fix when a site in the
family is 'closed' preventing new account registrations.