### Description

InternalComposite.formatObject() can throw an IllegalArgumentException if the format to be applied to a composite key leads to a loss of information (eg, a date histogram with a width of one day is then given a date format of 'YYYY-mm', meaning that multiple buckets from the same month would produce the same key).  This is currently only checked during serialization, meaning that we can do a great deal of work building aggregations only to then throw the result away at the end.  This also causes a problem for converting the `_search` endpoint to using chunked xcontent serialization, as the HTTP response code is the first thing sent back, and so errors that occur during later serialization don't get handled correctly.
server/src/test/java/org/elasticsearch/search/aggregations/bucket/composite/InternalCompositeTests.java
final CompositeKey key = createCompositeKey();
            if (keys.contains(key)) {
                continue;
            }
            keys.add(key);
            InternalComposite.InternalBucket bucket = new InternalComposite.InternalBucket(
                sourceNames,
                formats,
                key,
                reverseMuls,
                missingOrders,
                1L,
                aggregations
            );
            buckets.add(bucket);
        }
        Collections.sort(buckets, (o1, o2) -> o1.compareKey(o2));
        CompositeKey lastBucket = buckets.size() > 0 ? buckets.get(buckets.size() - 1).getRawKey() : null;
        return new InternalComposite(
            name,
            size,
            sourceNames,
            formats,
            buckets,
            lastBucket,
            reverseMuls,
            missingOrders,
            randomBoolean(),
            metadata
        );