Feature - Sane API error generation

Description

Checking for errors is common in API controllers. Right now almost all of these checks are an if statement returning a special Response value containing the list of errors generated during that call. This way of replying an error requires many similar blocks all over the codebase. This is not fine.

This problem is also one of the motivations behind #156.

Priority

High - The functionality impact is low, but the maintainability suffers terribly.

Definition of done

When it is possible to generate error responses in a more elegant manner, such as using exceptions.