Confd: don't fail if the config doesn't load

Rather than quitting we'll just continue to poll the config at a slow
rate, hoping that sooner or later we'll get it back. This allows also
working on non-MC nodes, and smoothly transitioning from MC to non-MC,
other than dealing with a few temporary breakages.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>