[#1247] object: Return NOT_FOUND and ALREADY_REMOVED statuses

Replace `ErrNotFound`/`ErrAlreadyRemoved` error from
`pkg/core/object` package with `ObjectNotFound`/`ObjectAlreadyRemoved`
one from `apistatus` package. These errors are returned by storage
node's server as NeoFS API statuses.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/local_object_storage/blobstor/get_small.go

@@ -18,6 +18,8 @@ type GetSmallRes struct {
 //
 // Returns any error encountered that
 // did not allow to completely read the object.
+//
+// Returns apistatus.ObjectNotFound if requested object is missing in blobovnicza(s).
 func (b *BlobStor) GetSmall(prm *GetSmallPrm) (*GetSmallRes, error) {
 	return b.blobovniczas.get(prm)
 }