v8.15.1

Fixes

Make string response body handling more consistent

Error responses whose error is a plain string (eg HTML from Cloudflare), with a status other than 400, 401, 403 or 429 (for example a 404 or 500), previously raised GoCardlessProException: Invalid error type and lost the message from the response. They now raise GoCardlessInternalException with the message from the response, and getType() returns gocardless as it is an internal error.