core : Display custom error page if reports is not installed

When reports is not installed the link from
/ovirt-engine/ page takes you to 404 page when
user clicks on reports link.

A custom error page is displayed that shows
the message that reports is not installed

Change-Id: Idaed902328695e2d448ac49c414ddfbde8d3fd25
Bug-Url: https://bugzilla.redhat.com/1143998
Signed-off-by: Ravi Nori <rnori@redhat.com>