[racket] online check syntax error display tweak

I've changed how online check syntax reports errors. Instead of
hilighting the source location in gold, it puts a little reddish thing
in the right-hand margin where the error is. The goal is to make this
less obtrusive in, say, lecture, but also to make it less painful for
those among us who edit programs in such a way as to keep the parens
always balanced.

Let me know if you think it is an improvement or not

Robby