Implement JSON error code handling

[kozabrada123]

from https://docs.discord.sex/topics/opcodes-and-status-codes#json:

Along with the HTTP error code, the API can also return more detailed error codes through a code key in the JSON error response. The response will also contain a message key containing a more friendly error string. Some of these errors may include additional details in the form of Error Messages provided by an errors object.

This can help narrow down the error, for example in POST /users/@me/mfa/totp/enable:

{
    "message": "Invalid two-factor secret",
    "code": 60005
}

vs

{
    "message": "Password does not match",
    "code": 50018
}

[bitfl0wer]

symfonia should also implement this. I am going to create an issue there