cli: consider files vanishing during notmuch new non-fatal

cli: consider files vanishing during notmuch new non-fatal

If some software other than notmuch new renames or removes files
during the notmuch new scan (specifically after scandir but before
indexing the file), keep going instead of bailing out. Failing to
index the file is just a race condition between notmuch and the other
software; the rename could happen after the notmuch new scan
anyway. It's not fatal, and we'll catch the renamed files on the next
scan.

Add a new exit code for when files vanished, so the caller has a
chance to detect the race and re-run notmuch new to recover.

Reported by Paul Wise <pabs@debian.org> at
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=843127