gmi fails to init if notmuch is set up with all defaults

[dbeecham]

To reproduce, remove the key path under [database] and the key tags under [new], then run gmi init. If you don't specify a database path or tags during notmuch setup, it uses the default values /home/you/mail and unread;inbox;.

[dbeecham]

Additionally, if the init of gmi fails, you're stuck with a partially initialized gmi directory.