Handle too_many_buckets_exception more gracefully.

[myronmarston]

Previously, we returned a 500 Internal Server Error which was not a great experience when it would consistently happen for a particular query. Now we return a nicely formatted error telling the client to reduce the grouping cardinality instead.

Here's an example, after I used kibana to update the max_buckets setting:

PUT /_cluster/settings
{
  "persistent": {
    "search.max_buckets": 100
  }
}