Every error passed to error presenter was wrapped make it hard to handle unexpected error

### What happened?

Currently, we have an `ErrorPresenter` to intercept the error response. But every error from the operation is wrapped with `ErrorOnPath`. This makes it hard to handle unknown errors and response raw error messages to the users.

https://github.com/99designs/gqlgen/blob/76d444c15fc1d98538648a229f41ed7b4573bad7/graphql/context_response.go#L52-L61

Another thing is if we want to strip some error out from the response by returning `nil` value from `ErrorPresenter` will cause the app to panic for now. I suggest checking for `nil` value before appending the error to `c.errors`.

And by default when we create a new handler `DefaultErrorPresenter` was set and it has the ability to wrap the error path then I think we don't need to call `ErrorOnPath` before passing an error to `ErrorPresenter`.

### versions
- `gqlgen` v0.17.35
- `go` v1.20.6