Improve input validation and error handling when parsing `.md` files

[Gedochao]

Version(s) nightly

Describe the bug The input validation for Markdown files is a bit naive right now. Only the contents of scala code blocks are validated, while the Markdown code as a whole is not.

To Reproduce

# Unclosed snippet
```scala

Expected behaviour Markdown files should be validated as a whole when they are compiled, and not just their scala code blocks' contents