This error occurs when `notmuch-fcc-dirs' is set to a list. The error
was in the `notmuch-fcc-dirs' format check which was changed in an
incompatible way from 0.4 to 0.5.
The fix was extracted from a bigger patch series by David
Edmondson id:"1290682750-30283-2-git-send-email-dme@dme.org".