Custom data source provider

I have some data that comes through a Web Service and it is cached locally
using the isolated storage. For me it seems like a good scenario for
implementing a custom data source provider: look for the data in the cache,
if the data is stale get it from the web service.

My questions are:
1. How is the error mechanism working? For example, if an exception is
raised during the connection to the web service, how do I notify about this
error? I have thought that Error property will automatically have a reference
to the exception raised during BeginQuery (something like asynchronous web
service error mechanism).
2. How about asynchronous retrieval? The base class DataSourceProvider
doesn't have built-in support for asynchronous data retrieval like
Object/XmlDataProvider have so it seems that I have to implement it myself.
--
Valentin Iliescu [MVP C#]