Exceptions are a way to implement the response to unexpected answers in a session.
A request comes with an expected type of answer. Usually the expectations include the number of returned resources in the selection.
An exception can be seen as a request for more information. You could for example request the phone number of a person with a certain name. The service finds two person with the specified name and tells you to choose whictch one you mean by throwing an exception. The exception will be picked up by one of the calling contexts. The context can answer by adding the extra information. If the information can't be found in the context, a response will be presented for the agent, requesting the additional criterion.
Not implemented