vmimage.subr: noisier failure for unsupported targets

The return code of write_partition_layout() doesn't bubble up, so an
invocation of make vm-release for an incorrect/unsupported target will
appear to succeed while make vm-install will fail due to missing
files. This isn't a common point of failure, but is worth handling
properly.

Upgrade this case to print a message to stderr, and exit in place. This
is okay to do since at this point in the execution of mk-vmimage.sh,
cleanup() has already been run.

Reviewed by:	gjb
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D26985

release/tools/vmimage.subr

@@ -43,8 +43,8 @@ write_partition_layout() {
 			ROOTFSPART="-p freebsd-ufs/rootfs:=${VMBASE}"
 			;;
 		*)
-			# ENOTSUPP
-			return 1
+			echo "vmimage.subr: unsupported target '${TARGET}:${TARGET_ARCH}'" >&2
+			exit 1
 			;;
 	esac