Compute: catch more specific exception for _get_instance_nw_info

There are cases when the method _heal_instance_info_cache tries to
update the network cache. The problem here is that the instance no
longer exists. The exception handling was very broad. This adds
treatment for the specific exception.

Change-Id: I04372cec81fde6ef06e9d7f2b59c1e47e15c0905
Closes-bug: #1385246
Merge "Compute: catch more specific exception for _get_instance_nw_info"