Log exception in _heal_instance_info_cache

Previously, if errors occurred in _heal_instance_info_cache() they would
go unnoticed as their log level was debug and the stace trace was not
displayed. This patch changes that so the exception is logged so periodic
failures here can be more easily detected and fixed.

Change-Id: I15bb3e7e29462e930f6159be9f4bbf4980b1270c
Closes-bug: #1257940
Merge "Log exception in _heal_instance_info_cache"