datasette --get exit code should reflect HTTP errors

[simonw]

If you run datasette . --get / and the result is a 500 or 404 error (anything that's not a 200 or a 30x) the exit code from the command should not be 0.

It should still output the returned content to stdout.

This will help with writing soundness checks, as seen in https://til.simonwillison.net/til/til/github-actions_grep-tests.md

[simonw]

I'm going to use exit code 1 for any errors, be they 500 or 404.