When a user doesn't have access to all the credentials of a job
template, they cannot copy the JT. However, currently we raise a
default `PermissionDenied`, which doesn't give the user insight
into what's wrong. So, this PR just adds a custom message indicating
that access to credentials is missing.
Signed-off-by: Vismay Golwala <vgolwala@redhat.com>