Handle HTTP return codes properly

[benoit74]

Fix #220

Changes:

• in the 2xx range,
  • only WARC responses with HTTP return code 200, 201, 202 and 203 are now supported
  • others now are simply ignored
• in the 3xx range
  • only WARC responses with HTTP return code 301, 302, 306 and 307 are supported if they redirect to a payload which is present in the WARC
  • they are replaced by a ZIM redirect entry
  • others are simply ignored
• all WARC responses with HTTP return codes in the 1xx (not supposed to exist in WARC files anyway), 4xx and 5xx ranges are now simply ignored

Adaptations are done at two places:

• to properly compute the list of ZIM entries which are supposed to exist
• to properly store the ZIM entries