Use EAFP approach in local.py

[sevdog]

This aims first of all to align the code of _CVar.__delattr__ with that of _Cvar.__getattr__.

Then to use the EAFP approach in Local._lock_storage, to improve efficiency since the hasattr method is implemented by catching the AttributeError of a getattr call.

I know that this should save just a couple of CPU cycle, but I belive it is worth it.