Stub app not resilient to missing main module

[freakboy3742]

Describe the bug

If an app is misconfigured and <app_name>.__main__ cannot be executed, the stub app fails almost silently.

Steps to reproduce

1. Generate a fresh "hello world" app
2. Delete the src/helloworld directory
3. Create an empty text file named src/helloworld
4. briefcase run
5. See error

[helloworld] Starting app...
===========================================================================
TypeError: 'str' object cannot be interpreted as an integer` error.

A similar error is raised if you delete the __main__.py file (which should generate an ImportError: No module named helloworld.__main__; 'helloworld' is a package and cannot be directly executed error).

However, if you delete app.py, the app correctly generates an error dialog. This may be because it is reported as ModuleNotFoundError: No module named 'helloworld.app', rather than an ImportError.

Expected behavior

An error message similar to the one raised by briefcase dev should be displayed.

[helloworld] Starting in dev mode...
===========================================================================
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "<frozen runpy>", line 222, in run_module
  File "<frozen runpy>", line 142, in _get_module_details
ImportError: No module named helloworld

Problem running app helloworld.

Screenshots

No response

Environment

• Operating System: Ubuntu 22.04
• Python version: all
• Software versions:
  • Briefcase: 0.3.20

Logs

Additional context

Originally reported as beeware/briefcase#2123, using a linux system backend.