feat: allow falling back to default error handler from custom error handler

[salmanm]

When custom errorHandler is specified in route config, it is responsible for handling all errors. But what if the custom error handler is interested in only some errors and wants to let fastify handle all other errors using it's default error handler.

This PR adds this feature.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

I might prefer to have an explicit:

const { defaultErrorHandler } = require('fastify')

So that developers can reuse our default implementation wherever they want.

[mcollina]

Can you add docs?

[mcollina]

lgtm

[jsumners]

LGTM

[mcollina]

lgtm

[github-actions]

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