Previously, something like 'guix import gnu which' would spit out a
backtrace if, say, the 'which' tarball could not be authenticated.
* guix/upstream.scm (download-tarball): Mention failure modes in
docstring.
* guix/import/gnu.scm (gnu-package->sexp): Return #f when
'download-tarball' returns #f.
* guix/scripts/import.scm (guix-import): Call 'leave' when IMPORTER does
not return a (package ...) sexp.
This is a followup to 0a7c5a0.
* guix/import/gnu.scm (preferred-archive-type): Use
'upstream-source-archive-types' instead of 'gnu-release-archive-types'.
(gnu-package->sexp): Use 'upstream-source-urls' et al. Update call to
'download-tarball'.
(gnu->guix-package): Use <upstream-source> instead of <gnu-release>.
* guix/upstream.scm (upstream-source-archive-types): Export.
Ludovic Courtès