core: explicit warning on live snapshot failure

On live snapshot failure, added an explicit warning stating
that using the created snapshot might cause data inconsistency.

Change-Id: Ib91c9d947a90e3b8fdd872bb3996dfb839e7767a
Bug-Url: https://bugzilla.redhat.com/1119203
Signed-off-by: Daniel Erez <derez@redhat.com>