Change the producers to return errors instead of failing and causing the process to exit

[lburgazzoli]

The current pattern to handle errors in producers is:

if err != nil {
    log.Fatal().Err(err).Msg("...")
 }

This causes to the os.Exit(1) function to be called.
In the light of better modularization/plugins, producers should return a proper error to let the caller a chance to handle it.

This means that the current Producer interface should be amended to become:

type Producer interface {
	Produce(ctx context.Context, k []byte, v []byte, o any) error
        Close(ctx context.Context) error
}

Relates to #163