CBLT-319:  FAIL if unable to download source package

Change-Id: Ib7634e3a3f2ac5518552d2c665f92e24be300830
Reviewed-on: http://review.couchbase.org/38562
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>