The Routes Preferred API returns error messages as part of the
response to a method call. For example, if you omit the API key from the
request, the method returns:

```json
{
  "error": {
    "code": 403,
    "message": "The request is missing a valid API key.",
    "status": "PERMISSION_DENIED"
  }
}
```

If you omit a required body parameter, such as `origin`, the method
returns:

```json
{
  "error": {
    "code": 400,
    "message": "Origin and destination must be set.",
    "status": "INVALID_ARGUMENT"
  }
}
```

For more information on errors and error handling, see
[Errors](https://cloud.google.com/apis/design/errors).