report the concrete types for unmatched prefab types instead of their aliases

[capfredf]

What version of Racket are you using?

8.0.0.5 on 3a49533ff52364684a6f7e2753e6811378f72008

What program did you run?

#lang typed/racket/base
(define val '#s(container 10))
(struct container
    ([value : String])
    #:prefab)
  (ann val container)

What should have happened?

Type Checker: type mismatch
expected: (Prefab container String)
given: (Prefab container Positive-Byte)
in: val

If you got an error message, please include it here.

Type Checker: type mismatch
expected: container
given: (Prefab container Positive-Byte)
in: val

[capfredf]

I will probably fix this along with #906