Update: Google says they've fixed the error, and we're no longer seeing it on our end.
The Google Contacts API has been sporadically returning errors (of type "503") for many of our customers over the last couple of weeks. When that happens you'll see an alert like this:
and, if you have "Show status in menu bar" selected, this icon in your menu bar:
The good news is the error isn't affecting any of your data and is sporadic. The bad news is the problem is entirely on Google's side, and there's not much we can do other than wait for them to fix it.
We've made Google aware of the issue and are optimistic they'll solve the problem quickly. In the mean time you can safely ignore these messages. The problem is sporadic, so if one sync fails chances are the next one will succeed. You can avoid seeing this alert altogether by disabling contact sync in the Spanning Sync pref pane.