Document possible extra fields in error responses

[godexsoft]

We are seeing that rippled produces extra fields in some error responses. This is not per specs and clio does not do the same (see https://github.com/XRPLF/clio/issues/660). @ximinez suggested we update the docs to specify that ledger_index and ledger_hash can be optionally added to error responses.

Example noripple_check response from rippled:

{ "result": { "error": "actMalformed", "error_code": 35, "error_message": "Account malformed.", "ledger_hash": "AEEF7A04B2F0448307CD4F94877CB326A86C56838ED42E67CC15F6F74B578453", "ledger_index": 29089522, "request": { "account": "r7x8JFCExiWopq7YuhK8AXjLhZe4fQ5e", "command": "noripple_check", "ledger_index": "validated", "role": "gateway", "transactions": true }, "status": "error", "transactions": [], "validated": true } }

[godexsoft]

This should be a general note in the errors section of the docs, not just for noripple_check. See https://github.com/XRPLF/clio/issues/406 👍