AP_Scripting: added pcall example

this shows how to catch errors in scripts to make them robust against
runtime errors

libraries/AP_Scripting/examples/protected_call.lua

@@ -0,0 +1,32 @@
+-- this shows how to protect against faults in your scripts
+-- you can wrap your update() call (or any other call) in a pcall()
+-- which catches errors, allowing you to take an appropriate action
+
+
+-- example main loop function
+function update()
+   local t = 0.001 * millis():tofloat()
+   gcs:send_text(0, string.format("TICK %.1fs", t))
+   if math.floor(t) % 10 == 0 then
+      -- deliberately make a bad call to cause a fault, asking for the 6th GPS status
+      -- as this is done inside a pcall() the error will be caught instead of stopping the script
+      local status = gps:status(5)
+   end
+end
+
+-- wrapper around update(). This calls update() at 5Hz,
+-- and if update faults then an error is displayed, but the script is not
+-- stopped
+function protected_wrapper()
+  local success, err = pcall(update)
+  if not success then
+     gcs:send_text(0, "Internal Error: " .. err)
+     -- when we fault we run the update function again after 1s, slowing it
+     -- down a bit so we don't flood the console with errors
+     return protected_wrapper, 1000
+  end
+  return protected_wrapper, 200
+end
+
+-- start running update loop
+return protected_wrapper()