webadmin: warn when exporting template version if base is missing

added a warning to the user, if trying to export template version,
and the base template is missing on the export domain,
because on import the base template must exist, or clone is needed.

this is very similar for exporting thin vm when template is missing.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1073035
Change-Id: I028f9e98a31cca391e05e2e02c535f5f1b2068cf
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>