Fixes #1377 - Handle non-existent interfaces better
Fixes #1377 - Handle non-existent interfaces better

If an interface is specified, but has no statistics, handle the
exception more gracefully.

guiniol/py3status,tobes/py3status,Andrwe/py3status,ultrabug/py3status,Andrwe/py3status,valdur55/py3status,tobes/py3status,ultrabug/py3status,guiniol/py3status,valdur55/py3status,valdur55/py3status,ultrabug/py3status