[Draft] Anchor: highlighting error locations

[asolove]

I would love some help understanding the error message tree that comes back from the compiler so that I can improve a couple cases where the Anchor UI shows message that don't clearly highlight the relevant source location.

Here's one case where the error has a loc but not a highlight and so it was showing very little detail on where the actual problem was. I've temporarily just copied a lot of the highlight-related code into this case, but I imagine there is a much better way to do this, either by changing the compiler error to emit a highlight in this case, or by some broader refactoring to how Anchor handles displaying srclocs.

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