Check for error in response to contact creation

[jmcx]

The server may return an error if the contact being created already exists. The client must integrate this response and inform the user.

[templth]

For such processing, you can throw a Restlet ResourceException within your server resources, as described below :

@Post
public Contact addContact(Contact contact) {
    if (contactExists(contact)) {
        throw new ResourceException(Status.CLIENT_ERROR_UNPROCESSABLE_ENTITY);
    }
}

Note that the Status.CLIENT_ERROR_UNPROCESSABLE_ENTITY corresponds the 422 status code.