registry: log errors on failure

If an image fails to be updated/created/deleted/etc for whatever reason,
the registry server returns 500 – but there's no trace of the actual
error. Let's catch the exception log it, and then re-raise it so it
still return a 500 status code. But at least we can debug.

Change-Id: Id437f23011a10365e85914dc1934f610bfa3dd28
VMware store.add to return the image size uploaded

This patch fixes a tempest test failing for the VMware datastore.
When the size provided to add() is zero but the image size is actually
larger than zero, add() should return the size of the image uploaded
and not zero.

Tempest failure:
api.image.v1.test_images.CreateRegisterImagesTest.test_register_then_upload

Change-Id: I16524c0f25aa4124c24ca25ac014819267dd72c3
Closes-Bug: #1292764
Merge "VMware store.add to return the image size uploaded"
Merge "registry: log errors on failure"