Errors in server function are printed as `_send_error_response` messages

When an error occurs in the server function, it gets printed to the console in a weird way. Here's an example app:

from shiny import App, render, ui

app_ui = ui.page_fluid("hello")

def server(input, output, session):
    raise ValueError()

app = App(app_ui, server)

INFO:     127.0.0.1:51018 - "GET / HTTP/1.1" 200 OK
INFO:     127.0.0.1:51018 - "GET /shiny-autoreload.js HTTP/1.1" 200 OK
INFO:     ('127.0.0.1', 51021) - "WebSocket /websocket/" [accepted]
INFO:     connection open
_send_error_response: this is an error

Also in shinylive:

A bisect traced it to #250:

*   1255ce1 Merge pull request #250 from rstudio/cpuinfo-example (7 days ago) <Winston Chang>
|\  
| * 73caa99 (refs/bisect/bad) Fix wasm (7 days ago) <Joe Cheng>
| * 799ed22 (refs/bisect/skip-799ed22d1ebf35eb6e5e51343c794ab031f8a89a) Fix connection issues (8 days ago) <Joe Cheng>
| * 66a55c5 (HEAD, refs/bisect/good-66a55c5e502fa16f883a152f97f57e1518a57fc3) Add example (9 days ago) <Joe Cheng>
* | 32edac6 Add reactive.poll to API reference (7 days ago) <Carson>
|/  
* 84df91e (refs/bisect/good-84df91ea8aab49b0d0775c73110c15bd40f38cca) Fix types (9 days ago) <Winston Chang>

Note that the previous behavior wasn't entirely correct either. The error only printed in the JS console (and not the Python console), and the app would not gray-out.

Start with the minimal Python app in the issue and reproduce the server-function error. Compare the behavior before and after the changes in #250, especially the bisected “Fix wasm” commit. Done means errors are reported appropriately without the _send_error_response message, and the app's error state behaves correctly in both the Python console and shinylive.