Failing plugin should not cause an empty host list to be returned

Changes the aggregation so that if a plugin is failing, the result
is ignored. This prevents the filtering engine from returning an
empty host list when something goes wrong.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1002444

Change-Id: Ie1a38e7007cd50f261290bf5f31b85cb9c8c6314
Signed-off-by: Martin Sivak <msivak@redhat.com>