Better messages for AttributeError in Shiny Express

[wch]

This closes #869.

Previously in the case of an AttributeError, it would print this:

ERROR:    Error loading ASGI app. Attribute "_2f_Users_2f_winston_2f_Projects_2f_py_2d_shiny_2f_examples_2f_express_2f_plot_5f_app_2e_py" not found in module "shiny.express.app".

With this PR, it will print a normal stack trace and then a RuntimeError:

  ...
  File "/Users/winston/Library/py-shiny/shiny/express/app.py", line 13, in __getattr__
    return wrap_express_app(Path(name))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/winston/Library/py-shiny/shiny/express/_run.py", line 44, in wrap_express_app
    app_ui = run_express(file)
             ^^^^^^^^^^^^^^^^^
  File "/Users/winston/Library/py-shiny/shiny/express/_run.py", line 129, in run_express
    raise RuntimeError(e)
RuntimeError: This is an error

This PR wraps the bulk of the run_express function in a try statement. I also added a bit of code to preserve the previous sys.displayhook, and then restore it in the finally. This is helpful when running multiple concurrent apps, to ensure that sys.displayhook isn't left in an intermediate state.

To see the actual substantive changes (and not just indentation change), turn on "Hide whitespace".

[wch]

Update: I added exception chaining (raise RuntimeError(e) from e), and now the output looks like this:

Traceback (most recent call last):
  File "/Users/winston/Projects/py-shiny/shiny/express/_run.py", line 98, in run_express
    exec(
  File "/Users/winston/Projects/py-shiny/examples/express/plot_app.py", line 17, in <module>
    render.asdf
AttributeError: module 'shiny.render' has no attribute 'asdf'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/Users/winston/Projects/py-shiny/venv/bin/shiny", line 33, in <module>
    sys.exit(load_entry_point('shiny', 'console_scripts', 'shiny')())
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/winston/Projects/py-shiny/venv/lib/python3.11/site-packages/click/core.py", line 1157, in __call__
    return self.main(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
 ...
 ...
  File "/Users/winston/Projects/py-shiny/shiny/express/app.py", line 13, in __getattr__
    return wrap_express_app(Path(name))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/winston/Projects/py-shiny/shiny/express/_run.py", line 44, in wrap_express_app
    app_ui = run_express(file)
             ^^^^^^^^^^^^^^^^^
  File "/Users/winston/Projects/py-shiny/shiny/express/_run.py", line 129, in run_express
    raise RuntimeError(e) from e
RuntimeError: module 'shiny.render' has no attribute 'asdf'