webadmin: Change warning message when exporting a VM without template

Change the warning message when exporting a VM based on template
to an export domain that does not contain this template to a more clearer
message

Change-Id: I559f867d6dfc2176d5324a8ff0b0163b0908840e
Signed-off-by: Tal Nisan <tnisan@redhat.com>
Bug-Url: https://bugzilla.redhat.com/964644