Marcelo Lopez Ruiz, a developer on the Astoria team, has written a nice post describing how to deal with Astoria server errors returned via HTTP.
One thing I regret that we didn't get into the Astoria December CTP was the ability to show detailed Astoria server errors to remote clients. Something like what ASP.NET supports. That way the developer can see what errors occurred on the server via the web browser. Currently, one has to have a debugger attached to the server process to get the equivalent information. This is particularity interesting in cases where the Astoria server code is not the component throwing the exception - for example when the Queryable provider throws an exception.
In addition, we are also looking at adding a way for providers to map specific exceptions to HTTP errors which could be used by provider writers to specify runtime HTTP errors instead of the generic "Internal Server Error". For example, if there was a database constraint error when inserting some data. Hopefully, this feature will make it into the next beta.