issue #447: Remove downloaded volume if an error occurs

If a storage volume is being downloaded by Kimchi and an error occurs
(e.g. timeout, network down, no available disk space), whatever has been
downloaded so far is kept on the storage pool.

Remove the partially downloaded storage volume if an error occurs during
the operation.

Signed-off-by: Crístian Viana <vianac@linux.vnet.ibm.com>