MB-100: handle all exceptions in _check_valid_keys

Change-Id: Ieaf31c88d618749f5099db070a55411d1a493bbe
Reviewed-on: http://review.couchbase.org/24777
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>