Using the subsystem layout for FW1 errors

I've been working on a FW1 application that uses subsystems, where I wanted the error messages (not that there will be any of course!) to show but use the correct layout for the current subsystem. Out of the box, FW1 will use the layout from defaultSubsystem to wrap around the error template (viewsmainerror.cfm). Setting the right layout for the subsystem that the error occured in turns out to be quite simple.