Reported by Andreas Enge <andreas@enge.fr> at <https://lists.gnu.org/archive/html/guix-devel/2015-11/msg00764.html>. * guix/import/pypi.scm (&missing-source-error): New error type. (latest-source-release): Raise it instead of using 'error'. (pypi->guix-package): Guard against it and use 'leave' to report the error. (latest-release): Guard against it and return #f when caught.