[Proposal] Error handler

# Error handler
- The main error handler spec: [error-handler.md](proposed/error-handler.md).
- Justification and other info: [error-handler-meta.md](proposed/error-handler-meta.md).
- See [ezzatron/php-error-handler](https://github.com/ezzatron/php-error-handler) for a reference implementation and full test suite.

The main spec is quite short, the meta document is quite lengthy. Please read through the meta document as it contains important concepts for the full utilization of the spec that are essential for productive discussion.