Function defaultErrorHandler

Fastify error handler supporting Boom errors and generic errors.

Negotiates the response format via @fastify/accepts:

• text/html — renders _error.ejs (requires @fastify/view).
• application/json — JSON payload.
• Otherwise — plain-text.

For Boom errors the HTTP status code and payload come from error.output. For generic errors a 500 is used unless the reply already has a 4xx/5xx status.