Make YAML loader be able to report multiple problems at once

[kantord]

Right now, there is a problem: the YAML loader fails at the first serious error, and does not attempt to analyze more files from the project.

This leads to annoying problems because - especially in the early stages of a course - people might be editing a lot of files at once and there could be a lot of problems present all at the same time. This will become extra annoying when the user realizes that after fixing a problem, a new problem will appear, and this process will be repeated until all problems are gone. There could be a large number of problems, and it could become overwhelming as without being able to see all problems at once, this can become overwhelming.

In order to avoid this issue we should change the YAML loader so that all problems are detected in one go, at least when this is technically possible.

[kantord]

One easy gain here could be, after the JSON schema validation is fully implemented, to validate all JSON files in one go in the beginning: #2438

That should be nearly trivial to solve.

Probably the majority of other errors is related to the mini-dictionary. That could require revamping how the analysis of courses and the handling of mini dictionaries is solved. The logic could be changed in a way so that first, all data can be loaded without analyzing the mini-dictionary. Then all errors not related to the mini dictionary could be reported here. After that, the handling of the mini dictionary could be implemented in such a way that all errors are reported