Hello!
You should actually be able to do this already. Inside Growl for
Windows, select the Trowl application, and select the Error growl
type. For Forwarding, make sure it is set to "Don't Forward". I had
my errors forwarded to iPhone for a while, but like you, it quickly
got annoying so I turned it off :)
Let me know if you have any problems doing this. Thanks for the
feedback!