fix list index error on plugins page when a manifest file does not exist

rhbz#1039233

Bug-Url:https://bugzilla.redhat.com/show_bug.cgi?id=1039233
Signed-off-by: Joey Boggs <jboggs@redhat.com>
Change-Id: I0d82583b3aee98b6b544ec89b75fa4f6ea314685