Fail translation if an invalid level is provided

[gitstashpop]

Setrans fails silently by returning the original label if a level cannot
be translated. Instead, go-setrans will now notify users if an error
occurs.

[gitstashpop]

@jbrindle I just realized that if you pass a valid pre-translated label to one of these calls, we'll now get an error. Is that desirable? Is this just something the caller should validate?

edit: we spoke offline, and decided to implement new functions with the desired behavior. That way we'll keep feature parity with libselinux/mcstrans with the existing functions.