Recover from wrong use of get-info :reason-unknown (#2667)

Recover from wrong use of get-info :reason-unknown (#2667)

Fixes #2584. Currently, we are immediately terminating CVC4 if the user
issues a `(get-info :reason-unknown)` command if it didn't succeed a
`(check-sat)` call returning `unknown`. This commit changes the behavior
to return an `(error ...)` but continue executing afterwards. It turns
the `ModalException` thrown in this case into a
`RecoverableModalException` and adds a check in
`GetInfoCommand::invoke()` to turn it into a
`CommandRecoverableFailure`, which solves the issue.