The assignment is written around a "happy path", and therefore does not mention the deviations from standard flow of execution. Ofcourse, in real life you will incorporate additional features as this one to make it more user friendly.
My recommendation is to get a basic design in place and then think about all the bells and whistles. You will not get extra points for additional features, and incorporating them may complicate your design. Perhaps you can think about them, and even design them, after you submit your necessary and sufficient solution.
Open Group Certified Distinguished IT Architect. Open Group Certified Master IT Architect. Sun Certified Architect (SCEA).