Move from netifaces to ifaddr and add better error handling

Display message if `owl` is not running (fixes #7, fixes #12)

setup.py

@@ -29,7 +29,7 @@ setup(
     package_data={
         'opendrop': ['certs/*.pem']
     },
-    install_requires=['requests', 'fleep', 'netifaces', 'Pillow',
+    install_requires=['requests', 'fleep', 'ifaddr', 'Pillow',
                       'requests_toolbelt', 'ctypescrypto', 'libarchive-c'],
     entry_points={
         'console_scripts': [