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.