In a recent post to her blog, Lorna Mitchell looks at giving feedback for errors on a web service.

Today we'll look at another really important aspect of authoring web services, and one feature that will definitely get used - error handling and feedback! Having clear and robust error handling in your service will help those trying to consume it immeasurably. Nothing is more annoying that impenetrable errors, unclear responses, or a service which accepts your input but then turns out not to have done what you expected. And I say that from experience.

She suggests that, instead of having one single error after another a more useful (and less frustrating) method is to have a "stack" of errors. Her code examples show how to added the errors to a $messages array and, at the end of the request, spit them back out in an easy to parse method.