MB-100: error instead of info where except in tasks

Change-Id: Id6d27fb01509c6759e86024fc38de08d8a270fa3
Reviewed-on: http://review.couchbase.org/24128
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>