The web crawl errors detailing "Errors for url in sitemaps" and "Not Found" have a new column, "Linked From" added by GWT. This "Linked From" provides linked pages/sources for the URL.
Here's an example from http://www.imavista.com ( Image Editing and Photo Retouching)
The figure shows 4 pages in the Linked From Column for Not Found URL. (click on image to see large view)
Now, after I clicked on 4 pages under "linked from", the above sources are shown. And the mentioned sources have broken links and so is under the Not Found Stat. Very much helpful information provided by "GWT /Linked From".