linux-initrd: Display a backtrace when the initial program fails.

* guix/build/linux-initrd.scm (boot-system): Add pre-unwind handler in
  'catch' form around 'primitive-load', and call 'format' and
  'display-backtrace' from there.

guix/build/linux-initrd.scm

@@ -482,10 +482,12 @@ to it are lost."
           (catch #t
             (lambda ()
               (primitive-load to-load))
+            (lambda args
+              (start-repl))
             (lambda args
               (format (current-error-port) "'~a' raised an exception: ~s~%"
                       to-load args)
-              (start-repl)))
+              (display-backtrace (make-stack #t) (current-error-port))))
           (format (current-error-port)
                   "boot program '~a' terminated, rebooting~%"
                   to-load)