Add worker.onerror handler to reject pending Promises

[bartadaniel]

Problem

When the Worker script fails to load (e.g. due to CORS/CORP policy, network error, or 404), new Worker() succeeds but the onerror event fires instead of onmessage. Since no onerror handler is set, all pending Promises — including the initial load() call — hang forever. They never resolve or reject.

This is a significant issue because:

• Callers have no way to detect the failure
• No error is surfaced — the operation silently hangs
• await ffmpeg.load() never returns, blocking the entire application

This affects any environment where the worker script URL might be blocked or unavailable (CORP headers, CSP, CDN outages, bundler misconfiguration, etc.).

Fix

Adds a worker.onerror handler in #registerHandlers() that:

1. Rejects all pending Promises (not just the load Promise) with a descriptive ERROR_WORKER error
2. Resets the internal worker state (#worker = null, loaded = false) so that load() can be retried by the caller

The implementation follows the same pattern used by terminate() for rejecting pending Promises.

Changes

• packages/ffmpeg/src/classes.ts: Add worker.onerror handler in #registerHandlers()
• packages/ffmpeg/src/errors.ts: Add ERROR_WORKER error constant

Testing

• TypeScript compilation passes with no errors
• The existing browser-based test suite doesn't have infrastructure for simulating Worker load failures. The fix is minimal and follows the same rejection pattern already used in terminate().

Fixes #532

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