Workflow Variables - Exception causes 'Display As ' to be ignored ?

I have several visible read/write Workflow variablesthat the user fills in during an Activity,

The userthen selects a Routing Eventand hits 'Complete Task'.

Transition code validates the user input and throws an exception with a nice error message if something is wrong.

Trouble is the Page is redisplayed and the variables displayed as their internal names NOT the 'Display As' strings.

Is there a trick I am missing ?

Ifit isthe exception that is causing this - then is their another way to pop up anerror message, without throwing an exception?

Any help much appreciated.