error-page ignored

I have webdefault.xml defined to include an error-page, like this:

<error-page>
<location>/error</location>
</error-page>

I have also tried this:

<error-page>
<error-code>500</error-code>
<location>/error</location>
</error-page>

and this:

<error-page>
<exception-type>java.lang.Throwable</exception-type>
<location>/error</location>
</error-page>

In all three cases, the exception stack trace is displayed in all its beauty, but not exactly what I'm looking for.