If you click on the Crawl tab on the side bar of GWT, you’ll see a link for Crawl Errors.

The Crawl Errors page lets you know these 3 site errors for the past 90 days:
• DNS errors
• Server errors
• Robots.txt failure

It will also let you know whether you have URL errors pertaining to Desktop, Smartphone, Feature Phone and Android Applications.

To diagnose your URL errors, you can click the Download button within the table to retrieve the top 1,000 errors and check each URL manually to see what’s wrong. If you don’t have that many URL errors, you can simply click on the link to see more details about the problem. Once you have addressed the error, you can checkmark the URL and select the “Mark as fixed” button.