Add not found schema to base services

This code is from the clojar badge:

```js
if (Object.keys(json).length === 0) {
/* Note the 'not found' response from clojars is:
status code = 200, body = {} */
throw new NotFound()
}
```

We have a bunch of other similar examples, where the schema is written to accommodate the not found case, and then there's code written to detect the alternate case and throw an error.

This responsibility is a kind of schema validation. We'd have clearer guarantees, particularly in the "found" case, if this responsiblity were delegated to Joi.

A really clean way to express this would be to optionally provide two schemas to `_requestJson` and friends. One would be the `schema` for the success case (like what we're passing now) and the other a `notFoundSchema`. If the `notFoundSchema` matches, we automatically throw `NotFound`, using the 404 error message.

No files or tests are named. Start by locating `_requestJson` and related base-service request helpers, then inspect existing schemas that handle not-found responses; done means an optional `notFoundSchema` is supported and matching responses throw `NotFound` with the 404 error message while success responses retain schema validation.

Written by the indexing model from the issue text.