Today we’re announcing more detailed Site Error information in Webmaster Tools. This information is useful when looking for the source of your Site Errors. For example, if your site suffers from server connectivity problems, your server may simply be misconfigured; then again, it could also be completely unavailable!
Site Errors will display statistics for each of your site-wide crawl errors from the past 90 days. In addition, it will show the failure rates for any category-specific errors that have been affecting your site.