Challenger should warn, not ban, humans browsing with cookies disabled

[graphiclunarkid]

Reproduction steps

1. Enable sha_inv challenge for a website.
2. Prevent that website from setting cookies by disabling them in browser (e.g. CSLite plugin).
3. Visit the website.

Current behaviour

The browser solves the sha_inv challenge and returns the result, but banjax bans the IP anyway, as it can't set a cookie. The user receives a 504 error and cannot access the website.

Proposal

Detect whether cookies are enabled, and if they are not but the browser solves the sha_inv successfully, inform the user that cookies are required instead of banning them (at least the first time).