If a contracted import doesn't have a pair value (because we haven't run the corresponding unit body yet), then result in a useful error instead of leaking internal implementation details.
If a contracted import doesn't have a pair value (because we haven't run
the corresponding unit body yet), then result in a useful error instead of
leaking internal implementation details.

svn: r17541

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