[PATCH] python: fix get_property error when property doesn't exist

[David Bremner]

VA <dev+notmuch at indigo.re> writes:

> Le 01/02/2019 à 14:45, David Bremner a écrit :
>> Should we be throwing an appropriate exception rather than returning
>> None? That seems more pythonic to me. In either case we should document
>> the error handling.
>
> What would it be? KeyError?
> Though, some standard Python methods like dict.get return None in case 
> the key doesn't exist.

Vincent pointed out on IRC that this patch doesn't introduce returning
None here, but just fixes the code that tries to do it. So this patch is
an improvement on the status quo.

d