pwclient: handle missing ~/.pwclientrc better

pwclient: handle missing ~/.pwclientrc better

The upgrade path will crash if ~/.pwclientrc doesn't exist, so add an
explicit check for the file before we try to "upgrade" it.

The default error messages don't mention the config file, so it can be
a bit confusing what pwclient is complaining about when running.

Signed-off-by: Mike Frysinger <vapier@chromium.org>
Acked-by: Brian Norris <computersforpeace@gmail.com>
Reviewed-by: Stephen Finucane <stephen.finucane@hotmail.com>