[PyWB2] Error page and status

[sebastian-nagel]

If an error occurs the index server responds with HTTP status code 200 OK, it should return a 503 or 5xx error. Seen with:

• call of non-existing API endpoint (collection) return 200 + empty result http://index-pywb2.commoncrawl.org/CC-MAIN-1970-12-index?url=commoncrawl.org&matchType=domain&output=json
• if index files are not publicly readable: returns 200 + text "Internal Server Error". Only uwsgi logs contain the error message/stack.

[sebastian-nagel]

Adressed in commoncrawl/pywb@93ef89e