This change allows clients to identify specific errors by an optional
code provided as part of any APIException instances raised in our code.

This code:

```py
raise ValidationError(
    "Please provide a correct date and time",
    code="time-invalid",
)
```

now returns an error object via the API with a message and a code
property making error processing on the client side easy for both humans
and machines and looks like this:

```json
{
    "message": "Please provide a correct date and time",
    "code": "time-invalid"
}
```