Add an option to downgrade errors to warnings

[Nadrieril]

In Charon we want to extract a crate even when there are errors. To achieve that, we use catch_unwind when translating any item. Except sometimes this isn't enough, because hax's use of struct_fatal and struct_err informs rustc that there was an error, which causes Charon to fail. This PR adds an option to avoid that.

[Nadrieril]

I found another solution: I can just ignore rustc's errors in Charon. So unless you have a use for this too, this is no longer useful.