Catch NotFound exception in http.Store get_size

Glance store doesn't catch NotFound exception in case image is not
found and it simply returns image size as 0.

Caught NotFound exception while getting the image_size if image
doesn't exist in store.

Change-Id: Ieb3dcb79ed9ba4368d0b3ca69b30095c1fd800b9
Partial-Bug: 1316137
Merge "Catch NotFound exception in http.Store get_size"