Distributed search: if all shards fail, fail the request despite shards.tolerant

If the request is tolerant to errors, logic in SearchHandler simply sets partialResults to true upon detecting an error in ShardResponse. This means a failure to query any shard would be reported back as partialResults=true, instead of a query failure.

Why: Improve the error handling and reporting in the search system by making sure that query failures are distinct from partial results