Set error message in erlang ffi

[stickyPiston]

The type of env.get is Result(String, String) but the erlang ffi does not return a string. So I added the same error message the js ffi returns to the erlang ffi. I'm not too familiar with erlang so it might not be as concise as it could be. It would be great if this could get merged, thanks in advance!

[aosasona]

@stickyPiston Great work! Thanks for catching this, in the future, it should ideally return an Option type since I will be supporting Deno, Bun and Node properly (which is the main reason we have a string error at all; to tell users when process.env is not available). The rewrite is slowly coming along but I will merge this and see if it can be improved, I do not know a lot of Erlang myself. Thanks again!