Newstreams

Small refactoring of Streams/Files:
- Added a simple error management system
- Removed FDStreams, reified under FStream

Next PR to come: Proposition for renaming of the classes of the stream hierarchy (as requested in #466)
protege avec un `if last_error != null then return`. Exemple: le open se passe mal, on veut pas que le close ecrase l'erreur du open.