core: better message on import template version

when trying to add a template version, which its base template is
missing, the message you get is not clear.

in this patch a better message provided.

Change-Id: I9fbf2c45c03fe9838bf25be5529c93c322372b28
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1073035
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>