rbd delete_image does not catch ImageNotFound when deleting snap

the store.rbd._delete_image() method does not catch rbd.ImageNotFound and return exception.NotFound when trying to delete a snapshot. The behaviour should be the same as when deleting the image itself.

This produces errors like this:

2013-09-30 14:30:10.139 442 ERROR glance.api.v2.image_data [0be083f3-4d9f-4d88-9264-b69ca9bbd2c2 18ab8e3ef26b499a8c581b8f18f2d33f 133f3a73a0294e98aaee00e8139bb922] Failed to upload image data due to internal error
2013-09-30 14:30:10.139 442 TRACE glance.api.v2.image_data Traceback (most recent call last):
2013-09-30 14:30:10.139 442 TRACE glance.api.v2.image_data File "/mnt/glance/glance/api/v2/image_data.py", line 55, in upload
2013-09-30 14:30:10.139 442 TRACE glance.api.v2.image_data image.set_data(data, size)
2013-09-30 14:30:10.139 442 TRACE glance.api.v2.image_data File "/mnt/glance/glance/domain/proxy.py", line 126, in set_data
2013-09-30 14:30:10.139 442 TRACE glance.api.v2.image_data self.base.set_data(data, size)
2013-09-30 14:30:10.139 442 TRACE glance.api.v2.image_data File "/mnt/glance/glance/notifier/__init__.py", line 202, in set_data
2013-09-30 14:30:10.139 442 TRACE glance.api.v2.image_data self.image.set_data(data, size)
2013-09-30 14:30:10.139 442 TRACE glance.api.v2.image_data File "/mnt/glance/glance/domain/proxy.py", line 126, in set_data
2013-09-30 14:30:10.139 442 TRACE glance.api.v2.image_data self.base.set_data(data, size)
2013-09-30 14:30:10.139 442 TRACE glance.api.v2.image_data File "/mnt/glance/glance/quota/__init__.py", line 140, in set_data
2013-09-30 14:30:10.139 442 TRACE glance.api.v2.image_data self.image.set_data(data, size=size)
2013-09-30 14:30:10.139 442 TRACE glance.api.v2.image_data File "/mnt/glance/glance/store/__init__.py", line 644, in set_data
2013-09-30 14:30:10.139 442 TRACE glance.api.v2.image_data self.image.image_id, utils.CooperativeReader(data), size)
2013-09-30 14:30:10.139 442 TRACE glance.api.v2.image_data File "/mnt/glance/glance/store/__init__.py", line 355, in add_to_backend
2013-09-30 14:30:10.139 442 TRACE glance.api.v2.image_data return store_add_to_backend(image_id, data, size, store)
2013-09-30 14:30:10.139 442 TRACE glance.api.v2.image_data File "/mnt/glance/glance/store/__init__.py", line 333, in store_add_to_backend
2013-09-30 14:30:10.139 442 TRACE glance.api.v2.image_data (location, size, checksum, metadata) = store.add(image_id, data, size)
2013-09-30 14:30:10.139 442 TRACE glance.api.v2.image_data File "/mnt/glance/glance/store/rbd.py", line 330, in add
2013-09-30 14:30:10.139 442 TRACE glance.api.v2.image_data self._delete_image(loc.image, loc.snapshot)
2013-09-30 14:30:10.139 442 TRACE glance.api.v2.image_data File "/mnt/glance/glance/store/rbd.py", line 267, in _delete_image
2013-09-30 14:30:10.139 442 TRACE glance.api.v2.image_data image.unprotect_snap(snapshot_name)
2013-09-30 14:30:10.139 442 TRACE glance.api.v2.image_data File "/usr/lib/python2.7/dist-packages/rbd.py", line 578, in unprotect_snap
2013-09-30 14:30:10.139 442 TRACE glance.api.v2.image_data raise make_ex(ret, 'error unprotecting snapshot %s@%s' % (self.name, name))
2013-09-30 14:30:10.139 442 TRACE glance.api.v2.image_data ImageNotFound: error unprotecting snapshot bccb3478-e7cf-4661-86ba-d405cffe0912@snap

where we are doing a cleanup but the image may not exist in which case we would want to ignore the error.