Render plugin mechanism needs `error` and `truncated` fields

While working on:
- https://github.com/simonw/datasette/pull/2118

It became clear that the `render` callback function documented here: https://docs.datasette.io/en/0.64.3/plugin_hooks.html#register-output-renderer-datasette

Needs to grow the ability to be told if an error occurred (an `error` string) and if the results were truncated (a `truncated` boolean).