MB-100: handle ValueError in BatchedValidateDataTask._check_valid_keys

Change-Id: I860a539616dfb50283f3eee40bcc648798cb7a83
Reviewed-on: http://review.couchbase.org/24264
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>