Exception handling strategy question

[remko de knikker]

when implementing a web service and a ws client, what is the general 
design principle for exception handling, when
1. when user entered a non-valid input value, for instance it doesn't 
exist in the database?
Do I simply through a SOAPFault or is it better to append my own 
<doesnotexist> tag?

2. as a client, do I catch an xxxxException/Fault, do I look for the 
soapenv:Fault tag or do I have to look the webservice specific <wserror> 
tag??

What is the best design strategy here??

thanks,
r