view should include an error response if some vbuckets are not considered or nodes are down

The behavior of views when some vbuckets or nodes are unavailable is currently unknown. The requirement is that at a minimum, view responses include an error at the end of the response (an example of which is in MB-4594).

We may also want to consider an additional view query param which will send back an HTTP error if the request can't be serviced (i.e. the cluster is unhealthy) or results don't come back from a given vbucket.