errors during opam file translation should be clearer #13784
Description
Errors that happen during opam file translation, such as:
File "packages/ocaml-compiler/ocaml-compiler.5.5.0~alpha1/opam", line 1, characters 0-0:
Error: Variable "build-id" is not supported.
Are strange because they have a file location for something the user doesn't see and don't provide a lot of context. Although we should be translating every opam file correctly, situations where we explicitly don't support variables may occur and we should at least try to have a better error message in this cases.