bsdinstall: Fail nicely in jail target

If the directory is empty we fail with a message regarding mkdir in which
the empty directory can't be seen because it is not quoted.

Show a nice message so the user knows what is going on.

Reviewed by:		bapt@
Differential Revision:	https://reviews.freebsd.org/D42252

usr.sbin/bsdinstall/scripts/jail

@@ -36,7 +36,6 @@ BSDCFG_SHARE="/usr/share/bsdconfig"
 : ${BSDDIALOG_OK=0}
 
 f_dprintf "Began Installation at %s" "$( date )"
-export BSDINSTALL_CHROOT=$1
 
 error() {
 	local msg
@@ -54,6 +53,11 @@ error() {
 	fi
 }
 
+if [ -z "$1" ]; then
+	error "Directory can not be empty\n\nUsage:\nbsdinstall jail directory"
+fi
+export BSDINSTALL_CHROOT=$1
+
 rm -rf $BSDINSTALL_TMPETC
 mkdir $BSDINSTALL_TMPETC
 mkdir -p $1 || error "mkdir failed for $1"