`shiny` script prints unhelpful message when import fails

If you add this to the top of examples/simple/app.py:

import asdf

Then launch with:

shiny run examples/simple/app.py

It will print this error message, which is confusing:

$ shiny run examples/simple/app.py
Traceback (most recent call last):
  File "/Users/winston/bin/shiny", line 33, in <module>
    sys.exit(load_entry_point('shiny', 'console_scripts', 'shiny')())
  File "/opt/homebrew/lib/python3.9/site-packages/click/core.py", line 1128, in __call__
    return self.main(*args, **kwargs)
  File "/opt/homebrew/lib/python3.9/site-packages/click/core.py", line 1053, in main
    rv = self.invoke(ctx)
  File "/opt/homebrew/lib/python3.9/site-packages/click/core.py", line 1659, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/opt/homebrew/lib/python3.9/site-packages/click/core.py", line 1395, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/opt/homebrew/lib/python3.9/site-packages/click/core.py", line 754, in invoke
    return __callback(*args, **kwargs)
  File "/Users/winston/Dropbox/Projects/prism/shiny/_main.py", line 98, in run
    app = resolve_app(app, app_dir)
  File "/Users/winston/Dropbox/Projects/prism/shiny/_main.py", line 155, in resolve_app
    raise ImportError(f"Could not find the module '{module}'")
ImportError: Could not find the module 'examples.simple.app'

Reproduce the failure with shiny run examples/simple/app.py, then inspect shiny/_main.py around run and resolve_app, the locations shown in the traceback. The issue does not specify the desired wording or handling, so clarify that first; done should mean import failures produce a more helpful message than the current module-not-found traceback.