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

[Ralph Seichter]

This is what happens when calling "notmuch dump" (version 0.29.1) with
an output file that is not writeable (tested with FISH and BASH):

  root > touch /tmp/out
  root > ls -l /tmp/out
  -rw-r--r-- 1 root root 0 Jul 22 20:36 /tmp/out

  nonroot > notmuch dump --output=/tmp/out
  Error renaming /tmp/out.kuZ9t5 to /tmp/out: Operation not permitted
  double free or corruption (!prev)
  fish: “notmuch dump --output=/tmp/out” terminated by signal SIGABRT (Abort)

While it is understood that Notmuch cannot write to the specified output
file, I don't think this should result in something as harsh as SIGABRT.

-Ralph