Verification errors mistreat the field `cached`

[aterga]

In particular, the method PartialVerificationError .withReason, which is implemented in many case classes all over src/main/scala/viper/silver/verifier/VerificationError.scala, needs to properly set the field cached. Otherwise, error transformations use withReason result in errors that might be actually cached with no way of checking that.

Thanks @marcoeilers for discovering this problem.