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.