"Fix" handling of old software versions on startup

Currently, masterd startup with old software versions is very confusing
for users: we present two tracebacks, with a message in the middle about
"version mismatch". This can lead to users believing that all that needs
to be done is to fix the config file.

This patch attempts to improve this by handling this case in masterd
itself (not in the child), and showing a more friendly message for this
case.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>