Bug: SIGABRT if "notmuch dump" output file is not writeable

[Ralph Seichter]

* David Bremner:

> I agree it's a bit ugly to look at.

Ah, euphemisms. ;-) Personally, I associate "double free or corruption
(!prev)" with memory trouble or situations where a library cannot
recover from an error state and needs to bail out using abort(). Not
being able to (over)write an existing file is not that serious, IMO.

> Do you see any database corruption or more serious issues?

No, and I don't expect any, as I am assuming that "notmuch dump" will
only ever read the DB.

-Ralph