### 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/main/java/org/elasticsearch/search/aggregations/bucket/composite/InternalComposite.java
&& Objects.equals(aggregations, that.aggregations);
        }
        @Override
        public Map<String, Object> getKey() {
            
            return new ArrayMap(sourceNames, formats, key.values());
        }
        
        
        CompositeKey getRawKey() {
            return key;
        }
        @Override
        public String getKeyAsString() {
            StringBuilder builder = new StringBuilder();
            builder.append('{');
            for (int i = 0; i < key.size(); i++) {
                if (i > 0) {
                    builder.append(", ");
                }
                builder.append(sourceNames.get(i));
                builder.append('=');
                builder.append(formatObject(key.get(i), formats.get(i)));
            }
            builder.append('}');
            return builder.toString();
        }
        @Override
        public long getDocCount() {
            return docCount;