arm64 make-memstick.sh: use 'set -e' to abort if any step fails

Also remove the now-redundant error handling that was only for makefs.

This script was run on an older FreeBSD host that lacked efi-on-mbr
support in makefs. A warning was emitted on the console (from makefs)
but the script continued running and exited with 0.

Reviewed by:	gjb
Sponsored by:	The FreeBSD Foundation

release/arm64/make-memstick.sh

@@ -10,6 +10,8 @@
 # $FreeBSD$
 #
 
+set -e
+
 PATH=/bin:/usr/bin:/sbin:/usr/sbin
 export PATH
 
@@ -31,10 +33,6 @@ fi
 echo '/dev/ufs/FreeBSD_Install / ufs ro,noatime 1 1' > ${1}/etc/fstab
 echo 'root_rw_mount="NO"' > ${1}/etc/rc.conf.local
 makefs -B little -o label=FreeBSD_Install ${2}.part ${1}
-if [ $? -ne 0 ]; then
-	echo "makefs failed"
-	exit 1
-fi
 rm ${1}/etc/fstab
 rm ${1}/etc/rc.conf.local