Manage REST status codes

[lampajr]

Is your feature request related to a problem? Please describe.
Right now the REST proxy is returning 200 or 201 if everything went fine and it is returning 500 in case of any error.
Example:
https://github.com/opendatahub-io/model-registry/blob/44d87e40327f8b4ac9ceac2a9013fe39807d051c/internal/server/openapi/api_model_registry_service_service.go#L168-L177

Describe the solution you'd like
Properly manage the returned errors, including 404 and so on.

I see this implementation a 2-step impl:

1. Adapt the core layer to return custom errors that already include the motivation (e..g, using some helpful enumeration)
2. Make REST checking the error returned by the core layer and return the proper code

Describe alternatives you've considered
n/a

Additional context

• Create custom errors: https://www.digitalocean.com/community/tutorials/creating-custom-errors-in-go
• HTTP statuses: https://go.dev/src/net/http/status.go