The Google Webmaster Central blog announced that the Site Errors report now is a lot more detailed and useful for webmasters. Now, the errors are broken down by each category into more specific errors.
For example, if your site is not accessible to GoogleBot, Google will try to say it is because of a DNS issue or server is down or maybe a robots.txt file preventing access. Google will also display statistics for each of your site-wide crawl errors from the past 90 days. And the report will show the failure rates for any category-specific errors that it finds for your web site.
Here is an example report:
If any of the errors do not make sense, Google will let you hover your cursor over the error name and Google will provide a short summary of the error, with a link to more details on the Google Help page.