Add connection verification to beginning of examples

[jason-watkins]

The example (for all languages) currently sends quite a lot of data without reading anything. As a result, it can appear that the example mostly works when X-Plane isn't even running.

It would probably be better to read a DREF or send another simple command that requires a response from the plugin. If this operation fails, we can immediately tell the user that something is wrong. This would be especially helpful for new users who have misplaced the plugin files.