[FEATURE] warn if ConversationHandler.states has a key -1

[DonutByte]

What kind of feature are you missing? Where do you notice a shortcoming of PTB?

I have now experienced the most frustrating side effect caused by ConversationHandler.END being set to -1.
I had a CH state key which was set to -1 and my entry point returned that state.
it caused the CH to exit (-1 is also END) right after my entry point which confused me.

here is an MWE which explains it better than I can with words:
https://pastebin.com/bk8MhwiX

to reproduce:
/start
pick any button - expected to continue the conversation, but instead the bot doesn't respond

Describe the solution you'd like

I thought maybe PTB can warn if -1 is set in ConversationHandler.states so nobody else has to experience it too :)
AFAIK there's no harm doing so because setting a state key to -1 is useless, it'll never get triggered

Describe alternatives you've considered

No response

Additional context

No response

[Bibo-Joshi]

I suggest to add this to #2770

[Bibo-Joshi]

Added to #2770. Closing this in favor of tracking everything in #2770