If you are using Tapestry 4, you can change your hivemodule.xml as follows:
<property name="exceptionPageName" value="ApplicationUnavailable"/>
Forgot to add that you'd also need to have a property or method named
"exception" in your custom exception page.
The property type is Throwable, which will be called by the Tapestry default
You can see an example of this from the Vlib example in Tapestry 4.