Callbacks into Toga invoked from .NET do not respect failure modes

[rmartin16]

Describe the bug

While investigating #2752, I discovered that the RuntimeError being raised because the WebView failed to initialize was swallowed and suppressed. Even running sys.exit(1) did not cause a visible error to be raised.

As such, developers and end-users are not informed of critical runtime errors.

Steps to reproduce

It's probably easiest to just force it; that is, just update winforms_initialization_completed to always raise an Exception and run an app using the WebView.

Expected behavior

Errors detected in such callbacks are reported to users.

Screenshots

No response

Environment

• Operating System: Windows 11
• Python version: 3.12
• Software versions:
  • Briefcase: 0.3.20.dev96+g23d1ed34
  • Toga: 0.4.6.dev321+g79719fce4

Logs

briefcase.2024_08_19-16_36_06.dev.log

Additional context

No response

[freakboy3742]

As a workaround, would it make sense to actually write the error message to the console, in addition to raising the RuntimeError and assuming it will be seen/surfaced? I agree this would be less than ideal, but it would be better to output something that indicates there's a problem rather than fail silently when we know there's been a problem.