If you have connected your domains to your Google Webmaster Tools, Google has some good news for you.
The company said that they would now alert webmasters if their domains are giving a significant number of crawling errors.
Since Googlebot regularly visits your site, we know when your site exhibits connectivity issues or suddenly spikes in pages returning HTTP error response codes (e.g. 404 File Not Found, 403 Forbidden, 503 Service Unavailable, etc). If your site is timing out or is exhibiting systemic errors when accessed by Googlebot, other visitors to your site might be having the same problem! When we see such errors, we may send alerts –- in the form of messages in the Webmaster Tools Message Center –- to let you know what we’ve detected. Hopefully, given this increased communication, you can fix potential issues that may otherwise impact your site’s visitors or your site’s presence in search.
These messages would be displayed in the Webmaster Tools dashboard. You can also enable email alerts in case you do not visit it frequently.