Throw new error when parsing YAML

[yanntrividic]

I started using Cosma with my Zettlr setup, and it was a bit frustrating not to know which file was the problem when there was a parsing error on the YAML frontmatter. Now it is more clear. I made it so it still stops the execution, but maybe it should be just a warning.

[infologie]

Thanks @yanntrividic. This is the first PR from outside the team! 🥳

@Myllaume and I had a look. For the end user, your modification is a net benefit, so I'm happy to merge. But first Guillaume will format the code so that it matches the rest of the codebase in terms of syntax.

This gave us the opportunity to think about a few things:

• There is a better way to handle this error. With your modification, modelize stops at the first YAML error. Ideally, we should catch them all then stop modelize and generate a report listing all the files with YAML syntax errors. I've added this to our task list.
• We need a proper error system to catch these things and present them to the user in an informative way. Guillaume is working on it but it takes time.
• We don't have guidelines for external contributions. We're going to work on that too.