Fix a bunch of :raises Error: syntaxes

Channeling @radix:

```
<radix> what bothers me most about sphinx is how poorly documented the whole ecosystem is
<radix> like, I really want *one* document describing the syntax of sphinx docstrings (and maybe
        another for the additional things available to the sphinx .rst prose format)
```

Another thing that's not very widely known is `:raises Error:`, not `:raises: :class:...`
Despite this being #666 I hope it won't actually break everything.