Patch for missing exception handling when using netifaces in rosgraph/network.py

We encountered a small bug when using the current ros_comm version in groovy. When netifaces.ifaddresses(...) is used on one of our CAN bus network interfaces, it throws an unhandled exception. This might actually be the driver's fault, but we cannot do anything about it right now. 

This occurred when roscore was launched, and it meant we couldn't launch anything on our robot. I tracked the source of the problem to a recent addition in rosgraph/network.py, and I simply added some exception handling/ignoring. Similar exception handling was done with netifaces in a different condition a few lines ahead of that addition. Unfortunately, I cannot test the change with many different configurations.