Make proposal filtering/validation consistent

[kkohbrok]

Currently we sometimes filter invalid proposals out and sometimes throw errors during validation. We should make this consistent. The simplest proposal would be to only do filtering with a first-come-first-serve approach.

[keks]

From reading the RFC, I think we should filter if the proposals contradict each other, and we should prefer proposals according to the recommendations of the RFC (e.g. this one). We should return an error if a proposal is outright invalid.

I think this that is what we do, or is it not?