installer: Fail if the bootloader could not be installed

If GRUB could not be installed during installation, the installer
continued without reporting the error to the user.

This change will make the installer fail.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>

src/installer/install-bootloader

@@ -172,12 +172,12 @@ function main() {
 	if device_is_mdraid "${device}"; then
 		local slave
 		for slave in $(mdraid_get_slaves "${device}"); do
-			grub_install "${slave}"
+			grub_install "${slave}" || return $?
 		done
 
 	# Handle normal block devices
 	else
-		grub_install "${device}"
+		grub_install "${device}" || return $?
 	fi
 
 	return 0