Add error HTTP status codes and fix error json marshal

[iansuvak]

Why this should be merged

How this works

writeJsonError didn't actually correctly serialize errors since the anonymous struct it was marshalling didn't have exposed fields so it would marshall to an empty struct.

Added status code to distinguish to the caller between user input errors and server side errors. Requests resulting in 5xx level errors can be retried and may succeed in the future while 4xx errors represent malformed or invalid input.

How this was tested

compiled the binary and manually tested against Fuji with common user supplied error cases. Got valid error reponses like:

➜  Code ./sendBadTx.sh
HTTP/1.1 400 Bad Request
Content-Type: application/json
Date: Fri, 20 Sep 2024 14:16:14 GMT
Content-Length: 36

{"error":"Could not decode message"}%

How is this documented

Updated the docs