Optional prefix message to exception, so we dont lose information from exceptions.  For example, in (module foo scheme/lang ...) we should still see the real error message.
Optional prefix message to exception, so we dont lose information from
exceptions.  For example, in (module foo scheme/lang ...) we should
still see the real error message.

svn: r10510

mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket