Implement structured errors for all user facing errors

[josephschorr]

Fixes #816

[jzelinskie]

For the most part, no major issues. I had some organizational questions, too, though.

[jzelinskie]

If everything implements the proper interface for producing metadata, I wonder if it's better to be explicit at the call-sites as to how we're converting. That way we can distinguish when two different calls can return the same error, but have different desired responses.

[josephschorr]

rewriteError is per package, so it shouldn't need to be differentiated (yet anyway)

[jzelinskie]

The only issue I see with this approach is it'll cause developers to just thoughtlessly do rewriteError(ctx, err) and not think about what actually needs to happen. We've been doing it up until now, so I think it's fine, but I definitely think this pattern promotes a lack of thought for properly handling errors.

[josephschorr]

Updated