Basic Bot should validate initial state

[SuuperW]

Summary

While looking at #4560 I encountered issues with Basic Bot failing to work due to improper setup. I think it should be able to detect these problems and tell the user:

1. The selected state/branch slot doesn't exist
2. Recording mode is turned off

In either of these situations, it will appear to just plain not work. If there is no savestate in the selected slot, there will be a HUD message about no state but this does not appear until after pausing or stopping the bot.

Host env.

BizHawk 2.11 dev