bug when setting custom error handler

I had a second look at it seems we have bugs in that part, before you even touched it.

https://github.com/fastify/under-pressure/blob/26677abff7385d274eb8836632d253e8fcc76abd/index.js#L107

When `opts.exposeStatusRoute.routeSchemaOpts` is in Object.assign before our changes, than it means we can not overwrite the error statuscodes.

The same here:
https://github.com/fastify/under-pressure/blob/26677abff7385d274eb8836632d253e8fcc76abd/index.js#L113

We could set a custom error handler like this:

```js
fastify.setErrorHandler((err, request, reply, done) => {
reply.status(err.statusCode)
.send({
code: err.code,
message: err.message
})
done()
})
```

a potential unit test would be:

```js
test('check error handler', async t => {
t.plan(1)
const fastify = Fastify({ exposeStatusRoute: true })

fastify.register(underPressure, {
healthCheck: () => {
throw new Error('Arbitrary Error')
},
exposeStatusRoute: {
routeOpts: {
logLevel: 'silent'
}
}
})

t.equal((await fastify.inject().get('/').end()).body, '{"code":"FST_UNDER_PRESSURE","message":"Service Unavailable"}')
})

```

_Originally posted by @Uzlopak in https://github.com/fastify/under-pressure/issues/198#issuecomment-1632101053_