Catch exception.InUseByStore at API layer

If user create an image with Ceph backend driver, and then create
an instance based on the image. As a result, user will run into
500 error when deleting the image. Because rbd driver is raising
exception.InUseByStore()at [1] when the image is in use, but no
code to catch the exception at REST API layer.

[1] https://github.com/openstack/glance/blob/master/glance/store/rbd.py#L290

Closes-Bug: 1345942

Change-Id: I350c235944aab226dc96de169be2087e2b599ab4
Merge "Catch exception.InUseByStore at API layer"