Rollup merge of #21980 - pnkfelix:more-robust-span-to-snippet, r=huonw

Rollup merge of #21980 - pnkfelix:more-robust-span-to-snippet, r=huonw

 This can be considered partial work on #8256.

The main observable change: macro expansion sometimes results in spans where `lo > hi`; so for now, when we have such a span, do not attempt to return a snippet result.

(Longer term, we might think about whether we could still present a snippet for the cases where this arises, e.g. perhaps by showing the whole macro as the snippet, assuming that is the sole cause of such spans; or by somehow looking up the closest AST node that holds both `lo` and `hi`, and showing that.)

As a drive-by, revised the API to return a `Result` rather than an `Option`, with better information-packed error value that should help us (and maybe also our users) identify the causes of such problems in the future.  Ideally the call-sites that really want an actual snippet would be updated to catch the newly added `Err` case and print something meaningful about it, but that is not part of this PR.