Documentation for 404.html, 500.html templates

[simonw]

This mechanism is not documented:

datasette/datasette/app.py

Lines 1119 to 1129 in 30b98e4

	templates = ["500.html"]
	if status != 500:
	templates = ["{}.html".format(status)] + templates
	info.update(
	{
	"ok": False,
	"error": message,
	"status": status,
	"title": title,
	}
	)

[simonw]

Also link to these from the docs added in #964.

[simonw]

In documenting this I realized that it's confusing that the default 500.html template is often used for non-500 errors (404 for example). I think I'll rename that default template to error.html instead.

[simonw]

Documented here: https://docs.datasette.io/en/latest/custom_templates.html#custom-error-pages