Remove ValidationErrors from visit_all_products

[tziemek]

I currently don't see why visit_all_products should return anything. We are just inspecting single products and always need context from the outside and populate some external collection.
As we don't break on the first occurrence of an error and always want to visit all products, I think we can simplify this method.
What are your thoughts? @milux @oxisto @peinjoh

[milux]

You might be right. If refactoring works, I see no reason not to do that simplification.
I think the return made more sense when we always returned on first error. With the current approach, probably not that much anymore.

[milux]

Same applies for the recursive branch walking then, I guess?

[tziemek]

Haven't look at it (you mean visit_branches_rec), but I guess so, as it is basically only used within the visit_all_products function.

[tziemek]

resolved in #170