Currently if you throw an error during any action, the response code is always 400. This change allows you to use the following pattern:

```
function NotFoundError(message, extraFields) {
    Error.call(this);

    this.name = this.constructor.name;
    this.message = message || '404: Not Found';
    this.responseHttpCode = 404;
    Object.assign(this, extraFields || {});
}
```

then in an action:

```
const { NotFoundError } = require('../../lib/customErrors');
...
const user = api.models.User.findByPk(userId);
if (!user) {
    throw new NotFoundError('Invalid userId');
}
```

This will result in a "Invalid userId" message passed back, with a responseHttpCode of 404 (not found). You can define as many error types as you want this way, e.g. using `SessionError/AuthError` for indicating 401/403 responses, `ServerError` for 5xx series errors (remote database failed), etc.