core: improve error message of export template

Improving the error message displayed in case of export template failure
- when one or more storage domains on which the template's disks are
located, are not available.
The fix does 2 things:
1. Fixes "Cannot export VM" to "Cannot export Template".
2. Rewrites second part of the message "Provided destination storage
domains doesn't match for provided disks." to a more clear and related
to scenario text.

related to bug  https://bugzilla.redhat.com/884780

Change-Id: Ib5b0bf022e6b8ee5502e30912d32a80fdd251d90
Signed-off-by: Alissa Bonas <abonas@redhat.com>