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

Nobody has claimed this yet.

> **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.

Start by reading the linked Gitter discussion and tracing Avram's form parameter handling, since the issue names no files or tests. Clarify the intended runtime and compile-time behavior, including when validation should occur, before identifying tests that define the completed error behavior.

Written by the indexing model from the issue text.