Proper Error objects with stack traces

[memburg]

Summary

Replace raw-value throw/catch with proper Error objects that include message and stack properties.

Motivation

throw and try/catch/finally are already implemented, but exceptions are raw values with no stack trace or structured error info. Proper Error objects would greatly improve debugging and align with JS expectations.

Scope

• Error constructor: new Error("message")
• Error.prototype.message
• Error.prototype.stack (string with stack trace)
• Error.prototype.toString()
• TypeError, ReferenceError, SyntaxError — use internally where appropriate
• Backward-compatible: raw-value throws should still work alongside Error objects

Out of scope

• Error.captureStackTrace
• AggregateError
• cause property / error.cause