core: detailed error message on OVF import error

The patch fixes the error message given when importing
a corrupted OVF file, added the actual exception message
to the audit log.

Change-Id: I8fdfd50fe4e956a95d05a70dac7391c39245c9f8
Bug-Url: https://bugzilla.redhat.com/1036635
Signed-off-by: Eldan Shachar <eshachar@redhat.com>