Fix Chat()'s loading message removal (on error)

[cpsievert]

After #1607, chat._remove_loading_message() isn't working as intended, which means the loading message doesn't get removed when an error is thrown at the start of a stream (or if a transform returns None). For a simple reprex, notice how the loading message here doesn't get removed when the error happens:

from shiny import reactive
from shiny.express import input, ui

ui.page_opts(
    title="Hello Shiny Chat",
    fillable=True,
    fillable_mobile=True,
)

chat = ui.Chat(id="chat")
chat.ui()


class FailingIterator:
    def __iter__(self):
        return self

    def __next__(self):
        raise Exception("Boom!")


@chat.on_user_submit
async def _():
    await chat.append_message_stream(FailingIterator())