Add more diagnosis for when "Send Slides" is not enabled

[lindon-fox]

There are multiple reasons why there might not be connected, it would be nice to get more information to shorten troubleshooting time.

I have failed to get it working on two different computers and I am not sure what the cause is.

[jpsim]

"Send slides" should be displayed whenever there's an active Multipeer Connectivity session. What kinds of diagnostics does MPC share that we could relay back to the user's screen?