Don't make such noise when ppp0 is unavailable.

On sholes ppp0 comes and goes with wifi and net availability.  We don't want our traffic
poller to have to care (and deal with race conditions) so just don't make so much noise
if we try to read the traffic data for a non-existing iface.

bug:2576057
Change-Id: I8c1d721e7afc288b08cffb79f8f52f6cab55c88b