Logging levels, process output and diagnostics for the MPF engine

[freezy]

Part of the logging and error reporting work. It follows the level policy from freezy/VisualPinball.Engine#590: errors are for VPE bugs, and problems the user or table author can fix are warnings. What the plugin knows goes into the session diagnostics that error reports carry.

• Unknown switch, coil and lamp numbers are warnings, once per number.
• MPF's stdout and stderr are always read when redirected (a full pipe could block MPF) and logged as "Mpf.Process", stderr by its own level.
• Startup failures say which step failed and where MPF's log is, and are reported through IGamelogicStartupStatus; rethrows keep their stack trace.
• The engine describes itself for error reports: MPF version, executable, machine folder, output mode, BCP state, settings.

Depends on freezy/VisualPinball.Engine#590 (the plugin API: IPluginLog, IDiagnosticsSource, IGamelogicStartupStatus), so merge that first.

[greptile-apps]

Confidence Score: 5/5

[Medium impact] The PR appears safe to merge after its stated plugin API dependency.

Summary

Adds clearer MPF logs and session diagnostics.

• MPF startup and process output now come with clearer details.
• MPF diagnostics now show the state and details behind a mismatch.
• Missing mappings and unknown MPF commands now get focused warnings.
• The included BCP media controller now explains connection and message problems.

Both previous findings are fixed. No new issues were found in the changes since the previous review.

_{Reviews (2) · Last reviewed commit: "logging: Keep warnings per table and ign..." · Reviewed by Greptile}