Fix callbacks behavior on save

## Primary change

Let's imagine a setup:

```ruby
class UserMapper < FlatMap::Mapper
  trait :registration do
    set_callback :save, :after, :notify_user

    def notify_user
      UserMailer.delay.send_registration_notification(target.id)
    end
  end
end

mapper = UserMapper.build(:registration)
mapper.apply(user_params)
```

At version `0.1.0`, `UserMapperRegistrationTrait`'s save routines are triggered *before* `UserMapper`'s ones. This means that target.id at this moment is `nil`, causing unpleasant errors. Such behavior is unexpected and this PR fixes that.

## Secondary change

`FlatMap::Errors#first_messages` method has been added for convenience, so we can call `render json: mapper.errors.first_messages`, for example. This may be useful in some scenarios.