gummiboot-postinst.sh: clarified error messages
gummiboot-postinst.sh: clarified error messages

android-ia/platform_external_gummiboot,freedesktop-unofficial-mirror/gummiboot,miguelinux/gummiboot2systemd-boot,FrozenCow/gummiboot,freedesktop-unofficial-mirror/gummiboot,liliang365/my_gummiboot,karelzak/gummiboot,todorez/gummiboot-multiboot2,trilean/gummiboot,liliang365/my_gummiboot,karelzak/gummiboot,miguelinux/gummiboot2systemd-boot,todorez/gummiboot-multiboot2,todorez/tummiboot,android-ia/platform_external_gummiboot,aaronp24/gummiboot,todorez/tummiboot,aaronp24/gummiboot,FrozenCow/gummiboot,trilean/gummiboot