Explore removing `update_or_keep/2` from Spandex.Span

[zachdaniel]

Right now, a lot of errors are ignored via usage of update_or_keep/2, which either updates the span and returns the new span, or doesn't update the span at all but returns the old span. This made some sense as a way to have consistent return types, but we're moving towards having more idiomatic return types, and that includes not swallowing errors. More discussion can be found in #63, where the issue was originally discovered. Additionally, that PR adds tests that can be un-skipped when the update functions have these new predictable return types.