[mapistore-python][sample.py: Enforce property usage instead of FID/MID attributes
[mapistore-python][sample.py]: Return MAPIStore errors instead of magic values

- Not necessarily a permanent solution - the MAPIStore Python bindings might stop being exposed in the backend at some point
[mapistore-python][sample.py]: Remove unnecessary return instructions
[mapistore-python][sample.py]: Remove __init__ in BackendObject (didn't do but a print)