nicely report any errors involving finding and using ifconfig

[zooko]

A couple of users have recently ​reported errors from ​iputil when it couldn't find their ifconfig executable, e.g.:

[Failure instance: Traceback: <type 'exceptions.OSError'>: [Errno 2] No such file or directory
/usr/lib/python2.7/threading.py:525:__bootstrap
/usr/lib/python2.7/threading.py:552:__bootstrap_inner
/usr/lib/python2.7/threading.py:505:run
- --- <exception caught here> ---
/usr/lib/python2.7/site-packages/twisted/python/threadpool.py:207:_worker
/usr/lib/python2.7/site-packages/twisted/python/context.py:118:callWithContext
/usr/lib/python2.7/site-packages/twisted/python/context.py:81:callWithContext
/home/kytv/tahoe/allmydata-tahoe-1.8.3/src/allmydata/util/iputil.py:222:_synchronously_find_addresses_via_config
/home/kytv/tahoe/allmydata-tahoe-1.8.3/src/allmydata/util/iputil.py:236:_query
/usr/lib/python2.7/subprocess.py:679:__init__
/usr/lib/python2.7/subprocess.py:1228:_execute_child
]
calling os.abort()

We should catch this exception and report a useful error message, saying what the sys.platform was and asking them to report about their ifconfig, like David-Sarah ​did on the list:

which ifconfig
ifconfig --help
ifconfig