Add a way to manage error on deletion

[Kmaschta]

Let's consider this configuration:

admin.errorMessage(function (response) {
    console.error(response);
    return response.data.error || JSON.stringify(response);
});

nga.configure(admin);

If an error is threw on deletion, it is not intercepted by errorMessage so response body is displayed to the user.

[fzaninotto]

Fixed by #982