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)
ajouter un reset du last_error ici. Cas debile mais possible: le close se passe mal mais le open se passe bien.

`last_error = null`