Display an error when trying to add an user with a non-localhost IP

[ghost]

Purpose

When trying to call add_api_user from a non-localhost IP, no error message is sent to the client, nor logged. This emits an error message using the custom error handling from #847.

Context

Emit an error when calling add_api_user from a remote IP.

Changes

Update the add_api_user method and update the API documentation with an output example.

How to test this PR

Calling the add_api_user from a remote IP should send back an error message. For instance curl -s "Content-Type: application/json" --data '{"method":"add_api_user","params:{"username":"toto","api_key":"toto"},"id":1,"jsonrpc":"2.0"}' http://10.0.0.1:5000/ | jq should return something like

{
  "id": 1,
  "error": {
    "message": "Unauthorized to call this method from a remote IP",
    "data": {
      "remote_ip": "10.0.0.2"
    },
    "code": -32603
  },
  "jsonrpc": "2.0"
}

[matsduf]

This is fine and works fine, but one comment on wording.

[matsduf]

Looks fine. Approved.

[matsduf]

Testing v2021.2

LGTM.