It means our robot was prevented from downloading this document. A robots.txt file is placed in the root of a website to limit access to that website by internet robots/crawlers/spiders, etc. People browsing a website protected by robots.txt are not affected by the file.
A http protocol error occured trying to request and download this URL
There was an ambiguous exception that occurred while trying to request and download this URL. Often this points to a DNS lookup failure. Could not resolve the hostname to an IP.
The robot tried and failed to download a document using a URL that was malformed. An attempt has been made to sanitize discovered URLs, but hey, nobody’s perfect.
The robot tried and failed to download a resource from a location without a protocol or with a mangled protocol. An attempt has been made to have the robot append ‘http://’ to schema-less URLs, but it is still possible to get this error.
This tool works over http and https protocols only. An attempt has been made to filter out all others, but it still may be possible to get this error.
The server either responded with too many redirects in a chain, or (more likely) is redirecting in a way that will never resolve.
A URL is required to make the connection to this document.
The request to the URL took too long and the robot gave up waiting
There was an error negotiating the https connection to this URL, due to a problem with the certificate or transport provider