onError Hook for EscrowMonitor

[connelevalsam]

closes #139

Summary
Implemented the onError hook for EscrowMonitor:
src/escrow/monitor.ts

• Added onError(callback) method + public types (EscrowMonitorOnError, EscrowMonitorErrorContext with phase: 'fetch' | 'handler', plus event/handler for handler failures) with typedoc comments.
• fetchFn failures: previously silently swallowed via .catch(() => []) — now try/catch that logs through logger.error and invokes onError(..., { phase: 'fetch' }).
• Handler failures: previously caught and logged to a simple logger — now invokes onError(..., { phase: 'handler', event, handler }).
• Existing consumers who don't register onError see unchanged polling/handler behavior.

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