Bulk: Failed preparsing does not fail whole bulk request

If a preparsing of the source is needed (due to mapping configuration,
which extracts the routing/id value from the source) and the source is not
valid JSON, then the whole bulk request is failed instead of a single
BulkRequest.

This commit ensures, that a broken JSON request is not forwarded to the
destination shard and creates an appropriate BulkItemResponse, which
includes a failure.

This also implied changing the BulkItemResponse serialization, because one
cannot be sure anymore, if a response includes an ID, in case it was not
specified and could not be extracted from the JSON.

Closes #4745