In Express mode, an `AttributeError` during `tagify()` results in unhelpful error

[wch]

Example:

from shiny.express import input, ui

with ui.nav_panel(title="One"):
    "one"

with ui.sidebar():
    "hello"

Results in this error:

ERROR:    Error loading ASGI app. Attribute "_2f_Users_2f_winston_2f_Projects_2f_py_2d_shiny_2f_examples_2f_express_2f_sidebar_5f_app_2e_py" not found in module "shiny.express.app".

[wch]

This error happens when an AttributeError is raised when a tagify() method is called on an object in the UI, and Uvicorn specifically looks for AttributeErrors and prints a customized error message.

It's very similar to #869, except that in that case, the AttributeError was raised during the initial evaluation of the code, whereas in this case, the AttributeError is raised later, when tagify() is called.

The running of tagify() happens here:

py-shiny/shiny/_app.py

Line 390 in 1ae8479

rendered = HTMLDocument(ui_res).render(lib_prefix=lib_prefix)