Surfacing more than just 1 error

[kengorab]

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Right now, if we encounter an error we immediately bail out and we're done. This worked for bootstrapping, but it would be a bit nicer if we could surface some more actionable error messages (and maybe even warnings!). If there's an error within a given block, we could record an error for that block, and then resume typechecking the code immediately following that block has completed.