Provide a warning if there's no final yield in an event handler generator function

[wwwillchen]

If you have an event handler like the following:

def on_click(e):
   do_work(e.key)
   yield
   do_more_work()

There's actually a bug where you should actually put a yield after do_more_work. Either: do_more_work doesn't get executed or it does get executed and you wait a while before the results are ready (not sure why the latter case occurs for on_load)

[wwwillchen]

It's difficult to do this programmatically (would require parsing AST), but we added docs on this point specifically.