Better errors processing

[zverok]

Follow-up of #8

• Not loose information from API:
  • API response: {"error"=>{"code"=>"gcmmissingparam", "info"=>"One of the parameters cmtitle, cmpageid is required", "*"=>"See https://en.wikipedia.org/w/api.php for API usage"}}
  • Response::Error currenty preserves only "info" field
• Maybe try to investigate/parse into errors; for ex., here we have gcm prefix in error code and cm-something params in message. In best case, MediaWiktory could say from it something like: params are missing for generator categorymembers, param list title, pageid and return this information structured. Or maybe it is completely superfluous :)