netaddr<=0.7.10 raises ValueError instead of AddrFormatError

This patch is based on Aarons work for the Bug#1308675.
It ensures that ValueError is also caught in addition to
AddrFormatError as in netaddr>=0.7.11 AddrFormatError is raised and
in netaddr<=0.7.10 ValueError is raised.

Change-Id: I71cfe82e57290552fb22637ad929747a6fa22460
Closes-bug: #1308675