This hides potentially confusing GRUB messages from the user, such as
"Installing for i386-pc platform."
* gnu/bootloader/extlinux.scm (install-extlinux): Use 'invoke/quiet'
instead of 'system*' and 'error'.
* gnu/bootloader/grub.scm (install-grub, install-grub-efi): Likewise.
* guix/scripts/system.scm (bootloader-installer-script): Guard against
'message-condition?' and handle them properly.