AP_Scripting: update analog input example to check for valid pin

libraries/AP_Scripting/examples/analog_input_and_GPIO.lua

@@ -6,7 +6,9 @@
 -- some are used by the main AP code, ie battery monitors
 -- assign them like this in the init, not in the main loop
 local analog_in = analog:channel()
-analog_in:set_pin(13) -- typically 13 is the battery input
+if not analog_in:set_pin(13) then -- typically 13 is the battery input
+  gcs:send_text(0, "Invalid analog pin")
+end
 
 -- load a input pwm pin
 local pwm_in = PWMSource()