Recent netaddr started to raise AttributeError for subnetpools values
like [{'start': ..., 'end': ...}].
The converter is meant to be graceful towards inputs that are not proper
ip addresses.
Instead of adding another exception type to the list and hope that the
list of expected types covers everything netaddr may raise, this patch
makes the converter catch all exceptions.
Change-Id: I967032c1dea690cd9da95ad3a8e382671186d971
d1020da3ea