Make exception handling in get_image_metadata more specific

The generic try/except with Exception was masking an
AttributeError raised during migration flows in conductor.

This change makes the error handling specific to the translated
exceptions raised from the image service in nova.image.glance,
specifically the _translate_image_exception method.

Related-Bug: #1326926

Change-Id: I3e4db73569fc1b02d7a05b4ab10fe83fec956922