Let "error" replication document updates bypass the VDU function.

This is necessary in the case where an software upgrade happens with
a more restrictive VDU function. Replicator db might end up having documents
which would not pass validation anymore, leading to a replicator manager
crash when it updates the document with an "error" state.

So in case of an "error" state allow malformed document so the user can
see the error.

COUCHDB-3199