add status value to _notmuch_message_ensure_metadata

[Gaute Hope]

Gaute Hope writes on februar 20, 2017 10:27:
> David Bremner writes on februar 18, 2017 15:45:
>> In id:1487339566.mz8acpov1j.astroid at strange.none , Gaute provided a
>> traceback of an uncaught Xapian::DatabaseModifiedError. The fix for
>> this is simple, but somewhat intrusive.
>>
>> [...]
>>
>> I haven't tested against Gaute's test case (needs more boost than I
>> have handy).

Alright then, attached is a non-boost version that takes a notmuch db
path (absolute) as the first argument (no warranty).

- gaute
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