Get rid of NO_INTERFACES_FOUND - it's not an error.

It just means "pcap didn't give me any interfaces, and didn't report an
error".  Hopefully, in the future, there will be pcap APIs that
distinguish between the (admittedly unlikely, these days) case of "there
really *are* no interfaces on which *anybody* can capture" and "you
don't have sufficient permission to capture", and we can report the
latter as an error.  (Given that pcap supports more than just "regular
interfaces", though, there are cases where you don't have permission to
capture on those but you have permission to capture raw USB traffic, for
example, so perhaps what's really needed is per-interface indications of
permissions.)

Change-Id: I7b8abb0829e8502f5259c95e8af31655f79d36a1
Reviewed-on: https://code.wireshark.org/review/3169
Reviewed-by: Guy Harris <guy@alum.mit.edu>