Capture "caused_by" in errors

If one attempts to index a very long value in a `flattened` field, it will cause indexing to fail. This is reported by `go-docappender`, but the reason for the error is obscured. It will look something like `"reason":"[1:40656] failed to parse field [error.exception.stacktrace] of type [flattened] in document with id '5UtbPI8BaMhqs2kExrLc'.`. Elasticsearch responds with a preview of the field value, but we strip this off.

The Elasticsearch response would also include "caused_by", which has helpful information about what caused the indexing to fail. In the example scenario above, it would be something like `"caused_by":{"type":"illegal_argument_exception","reason":"Flattened field [error.exception.stacktrace] contains one immense field whose keyed encoding is longer than the allowed max length of 32766 bytes. Key length: 17, value length: 121034 for key starting with [vars.request_body]"}`.

We should consider logging the cause to assist in debugging.