Raise nicer error if no nested params are in the params sent to a form

[paulcsmith]

This still needs some thought. Please don't work on this yet

It can be confusing when using forms in this scenario

https://gitter.im/luckyframework/Lobby?at=59f61f2d5a1758ed0f645e2f

After further thought, I think it should raise a helpful error at runtime, but ALSO raise a compile time error if you try to pass it params and have no allowed any fields

Also consider user params.nested? instead of params.nested! and only raising when you actually try to save. That way you can instantiate a form without nested params, but when you save it, it'll give you an error if the form name is not there.