Google has now added error reporting to the structured data report in their Webmaster Tools, which they announced will provide "insights into structured data errors on your website." The error reports are currently available only to webmasters who previously opted to participate in this Webmaster Tools trial, and Google will be soliciting feedback from the "trusted testers group" in the next few weeks.
Of the new report (found in the "Structured Data" section under "Optimization") Google says:
The errors shown in the reports are ones which prevent Google from displaying rich snippets. When errors are found, you can start by drilling down to a specific item type to see all the affected URLs. Clicking on a URL will then provide more detailed information about the items found and let you check it’s current status using the structured data testing tool. We hope this tool helps you to discover and fix incorrect structured data on your website.
And that's precisely how the new error reporting works. That is to say, the error reporting takes the form of a notification on the main page of the structured data report, as with these pages containing schema.org WebPage errors.
When you click on a data type with pages that contain errors, the nature of the error for each page is displayed in the report.
Details about the page that appear in the overlay when your click on a URL do not include any further information about structured data errors. Instead (as before) a link is provided that shows details about the page in Google's Structured Data Testing Tool, including the errors indicated in the Webmaster Tools Report.
I don't believe that there's any information provided in Webmaster Tools that isn't provided in the Google Structured Data Testing Tool. However, the addition of error reporting for structured data in Webmaster Tools is extremely useful, as it notifies webmasters of problematic markup without requiring webmasters to first validate their code in the Testing Tool.