core: Propagate the exception from VDSM to the user

Propagate the exception got from VDSM instead replacing it every time.

Change-Id: Ief319b187950fe188b54aad240e59cda807c0ef9
Related to Bug-Url: https://bugzilla.redhat.com/1037441
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>