Add warnings to modem scripts if the modem is not present

scripts/modem/sxmo_modeminfo.sh

@@ -1,6 +1,14 @@
 #!/usr/bin/env sh
+
+err() {
+	echo -e "$1" | dmenu -fn Terminus-20 -c -l 10
+	exit
+}
+
 modem_n() {
-  mmcli -L | grep -oE 'Modem\/([0-9]+)' | cut -d'/' -f2
+  MODEMS="$(mmcli -L)"
+  echo "$MODEMS" | grep -oE 'Modem\/([0-9]+)' > /dev/null || err "Couldn't find modem - is you're modem enabled?"
+  echo "$MODEMS" | grep -oE 'Modem\/([0-9]+)' | cut -d'/' -f2
 }
 
 st -e sh -c "mmcli -m $(modem_n) && read"