Refactor error handling

[nrc]

Should make it possible to add JSON or HTML errors. Also tidies up a lot.

[nrc]

r? @nikomatsakis

[nikomatsakis]

Travis is still unhappy but I can't quite see why. Still, this looks fine -- mostly just shuffling things around, from what I can tell. r=me once travis errors are resolved.

I feel like the layering of emitters/handlers/etc always feels more complex (or maybe just less clear) than it seems like it needs to be, but that's certainly pre-existing.

[nrc]

The idea (aiui) is that to implement a new way of dealing with errors, you just have to implement Emitter (which is just a single emit method, really). Handler presents a much wider interface to users (span_err, span_warn, etc.). I guess this is kind of convenient, but it won't really work with structured errors, so the next step is probably to make all of those default methods on the Emitter trait, at which point there is not much left to Handler and it can probably go.

[nrc]

@bors: r=@nikomatsakis

[bors]

📌 Commit 0f1ae7b has been approved by @nikomatsakis

[nrc]

@bors: r=@nikomatsakis

[bors]

📌 Commit 0f1ae7b has been approved by @nikomatsakis

[bors]

☔ The latest upstream changes (presumably #30206) made this pull request unmergeable. Please resolve the merge conflicts.

[nrc]

@bors: r=@nikomatsakis

[bors]

📌 Commit ff0c74f has been approved by @nikomatsakis