Ability to customize error messages

[simonw]

The error messages this produces aren't exactly user-friendly:

asgi-csrf/asgi_csrf.py

Lines 146 to 150 in f1a7e28

	await send_csrf_failed(
	scope,
	wrapped_send,
	"form-urlencoded POST field did not match cookie",
	)

asgi-csrf/asgi_csrf.py

Lines 168 to 188 in f1a7e28

	await send_csrf_failed(
	scope,
	wrapped_send,
	"multipart/form-data POST field did not match cookie",
	)
	return
	except FileBeforeToken:
	await send_csrf_failed(
	scope,
	wrapped_send,
	"File encountered before csrftoken - make sure csrftoken is first in the HTML",
	)
	return
	# Now replay the body
	await app(scope, replay_receive, wrapped_send)
	return
	else:
	await send_csrf_failed(
	scope, wrapped_send, message="Unknown content-type"
	)
	return

[simonw]

This came up because Datasette serves confusing messages and I'd like to be able to change those to include links to the relevant documentation: https://docs.datasette.io/en/stable/internals.html#csrf-protection