OP_QUERY style find accepts an option called partial, documented here. If set, mongos will return partial results when a shard is down rather than returning an error to the client.

In order to support this flag in 3.2, we need to do the following:

1. The find command should accept a parameter called allowPartialResults.
2. The AsyncResultsMerger in the new mongos read path should avoid propagating the error if only some of the shards can be contacted and allowPartialResults is set.