Handle transaction execution payload errors

[HDauven]

Closes #72

Summary

• interpret the current w3sper/RUES execution shape at event.payload
• treat payload.err and payload.error as transaction failures, including boolean error flags
• centralize execution-event parsing for extension and Tauri runtimes
• retain compatibility with legacy top-level and nested result event shapes
• preserve a populated compatibility error message when err is false or empty

Test plan

• npm run test:run (490 tests)
• npm run build:chrome
• npm run build:firefox