reconfigure: Correctly re-throw SRFI-34 exceptions on Guile 3.

Previously, we'd just print an ugly backtrace when running on Guile 3
because the '%exception throw would not be caught anywhere.

Reported by Arne Babenhauserheide <arne_bab@web.de>
in <https://bugs.gnu.org/40496>.

* guix/scripts/system/reconfigure.scm (install-bootloader-program): In
'catch' handler, match '%exception and use 'raise-exception' instead of
'throw' to rethrow in that case.