When Googlebot crawls your site, and encounters a response code other than "200" like a "404," or "disallowed by robots.txt," etc. whenever unable to reach your site content, this information is displayed in the Crawl errors section of Webmaster Tools.
"Google now sending "SiteNotice" message when detect a significant increase in number of crawl errors impacting a specific site. These notifications are meant to alert you of potential crawl-related issues and provide a sample set of URLs for diagnosing and fixing them. We hope you find SiteNotices helpful for discovering and dealing with issues that, if left unattended, could negatively affect your crawl coverage," explains Google.