Add truncation warning

[vadi2]

Add truncation warning, as it can be kind of rough to find this through trial and error or by noticing the context numbers.

[turboderp]

I don't think this is the right approach. Truncation is now a regular part of how the context window is adjusted, so it would spam hundreds of lines in the console. I've been meaning to add a feature to the UI that will visually indicate how much of the history is being used as context, but I haven't gotten around to it, and the front-end code probably needs to be cleaned up a bunch first.

[vadi2]

That approach would work just as well. Some kind of a return notification on the API would be nice too!