netaddr<=0.7.10 raises ValueError instead of AddrFormatError

This patch 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: I595c90e42129a2d365f3860e3042e826bd031365
Closes-bug: #1308675
(cherry picked from commit dd2ccd12adfb6c7d4aa500ba4120f767beeed35c)