* guix-download.in (http-fetch): Leave with an error message when
RESPONSE's code is not 200.
* tests/guix-download.sh: New file.
* Makefile.am (TESTS): Add it.
* guix/ui.scm: New file.
* Makefile.am (MODULES): Add it.
* po/POTFILES.in: Add it.
* guix-build.in: Use it.
(_, N_, leave): Remove.
(guix-build): Use `with-error-handling' instead of the `guard' form.
* guix-download.in: Use it.
(_, N_, leave): Remove.