Common error codes:
200 - OK / no error
301 - Redirect
302 - Redirect
400 - Bad request
401 - Unauthorized. This page requires a login+password.
402 - Payment Required
403 - Forbidden.
404 - Page not found
405 - Method Not Allowed
406 - Not Acceptable
407 - Proxy Authentication Required
408 - Request Timeout
409 - Conflict
410 - Gone
411 - Length Required
412 - Precondition Failed
413 - Request Entity Too Large
414 - Request-URI Too Long
415 - Unsupported Media Type
416 - Requested Range Not Satisfiable
417 - Expectation Failed
500 - Internal Server error
501 - Not Implemented
502 - Bad Gateway
503 - Service Unavailable
504 - Gateway Timeout
505 - HTTP Version Not Supported
0 - Some network connection error. Most likely it is a time out. Don't worry about it if it happens with a few pages.
990 - Blocked by your robots.txt file.
999 - Undefined error. Something went wrong, but we don't know what.
