feat: API errors now come with an optional code

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"
}
```