CSV ?sql= should indicate errors

[simonw]

https://latest.datasette.io/_memory.csv?sql=select+blah is a blank page right now:

curl -I 'https://latest.datasette.io/_memory.csv?sql=select+blah'

HTTP/2 200 
access-control-allow-origin: *
access-control-allow-headers: Authorization, Content-Type
access-control-expose-headers: Link
access-control-allow-methods: GET, POST, HEAD, OPTIONS
access-control-max-age: 3600
content-type: text/plain; charset=utf-8
x-databases: _memory, _internal, fixtures, fixtures2, extra_database, ephemeral
date: Mon, 07 Aug 2023 23:12:15 GMT
server: Google Frontend

Originally posted by @simonw in #2118 (comment)

[simonw]

This may end up being a renderer plugin hook design question too.