Simplify error handling in golang snippets

[ok300]

As discussed in #122 (comment) , this PR updates the error handling in golang snippets.

[ok300]

@JssDWt is this what you had in mind?

[dangeross]

For the functions where an err is returned, you need to add the return type to the function and return nil otherwise e.g.

func DoSomething() error {
    res, err := DoIt()
    if err != nil {
        return err
    }
    return nil
}

or if there is already a return type:

func DoThing() (*Thing, error) {
    ...
    if err != nil {
        return nil, err
    }
    ...
    return thing, nil
}

[JssDWt]

@JssDWt is this what you had in mind?

That's exactly what I had in mind!

[JssDWt]

Replaced by #183 because it was easier to recreate than to fix the conflicts.