errors: restructure formatparse() to clarify conditions a bit The `similar` list will be calculated only for `error.UnknownIdentifier`. It was then printed only if `inst.location is None`, which is true for that exception type, but it's an indirect condition to rely on. Also, it looked from the code like it could both report similarities and print a hint. That would be a little awkward because the similarity report looks similar to the hint (both are printed within parentheses). I also added a `elif` to clarify that. I plan to refactor this more coming patches so the similarity report actually is a hint. Differential Revision: https://phab.mercurial-scm.org/D9343