Better validation handling for FlowBundle

[zachbadgett]

Currently the 'Coordinator' in FlowBundle will throw a NotFoundHttpException if a Scenario is not valid. This limits flexibility and can be handled more effectively.

I'm willing to take care of it if we have an agreed solution. The best way I can think of is to have a ProcessValidator class available for extending or constructing that can be passed to the builder. If we want to keep the simplicity of the closure as well, we can transform it to the default validator. If a Scenario is invalid we can return a response using a selected or default error template; we can also redirect with a set flash message. Thoughts?

[pjedrzejewski]

I love this idea... I believe your idea with ProcessValidator is best what we can do now. Can you provide a PoC?

[pjedrzejewski]

Implemented by Zach.