add log for _get_images method

The _get_images method in glance/registry/api/v1/images.py doesn't
give logs when catch exceptions. Two logs for two different
exceptions will be very useful for debug.

Change-Id: I5725da0dafdb9909de1c31734b3c1c225483ca66
Closes-Bug:#1263871
Merge "add log for _get_images method"