testing/quick: improve function signature error.

testing/quick: improve function signature error.

This commit fixes a cosmetic defect whereby quick.Check reports that
the provided function returns too many values when it may, in fact,
return too few:

  func f() {}

  func TestFoo(t *testing.T) {
    if err := quick.Check(f, nil); err != nil {
      t.Fatal(err)
    }
  }
  // yields
  // $ go test -v foo_test.go
  // === RUN TestFoo
  // --- FAIL: TestFoo (0.00s)
  // 	foo_test.go:76: function returns more than one value.

Change-Id: Ia209ff5b57375b30f8db425454e80798908e8ff4
Reviewed-on: https://go-review.googlesource.com/11281
Reviewed-by: Russ Cox <rsc@golang.org>