docs: Update Error Handling section of `chat_append()`

[gadenbuie]

I missed that ?chat_append() included instructions for handling the rejected promise. (In my defense, I don't think I'm alone in this.)

shinychat/R/chat.R

Lines 172 to 180 in 98973d3

	#' # Error handling
	#'
	#' If the `response` argument is a generator, promise, or promise generator, and
	#' an error occurs while producing the message (e.g., an iteration in
	#' `stream_async` fails), the promise returned by `chat_append` will reject with
	#' the error. If the `chat_append` call is the last expression in a Shiny
	#' observer, Shiny will see that the observer failed, and end the user session.
	#' If you prefer to handle the error gracefully, use [promises::catch()] on the
	#' promise returned by `chat_append`.

The general idea is still correct, except that the error no longer brings down the app.