[plt-scheme] error message for case-sensitivity

My 12 year old just practiced some Scheme (second time) and I noticed 
how he spelled (draw-solid-disk ...) as (Draw-solid-disk ...). There 
were about 10 lines in his definition window, and he had added five 
since the last time he had clicked on execute. I was hoping to see how 
he'd react to the error message. Just before he was about to click on 
execute again, he said "oh, I see a mistake" and fixed it, to my 
disappointment. After thinking about it for a while, I realized that 
his 4th grade lessons in non-spelling had paid off. He goes to one of 
those American public schools that find it fashionable to focus on 
capitalization when it comes to spelling and nothing else. So seeing a 
capital letter scared him and he went lower case.

Just an observation about novices. -- Matthias

P.S. Our error message are over the head of any 12 year old. I am just 
waiting for Ollie to grow into this age group or for Robby to have kids 
so that we see some improvements in this area :-)