[Seaside] exceptions while debugging - frozen UI

[Holger Kleinsorgen]

Hello,

the following freezes the Squeak UI:

- implement a callback as "self halt zork"
- trigger the callback in the webbrowser (e.g. click on an anchor with 
this callback)
- the debugger pops up
- click on the callback method, step over halt, step over zork
- the web browser will now show the error in the usual way
- press debug
- UI is frozen, image is still alive