Improve configuration devex

[imsnif]

Right now, when we create a configuration for Zellij it's often hard to know what is expected of us. Part of this is because of the unforgiving nature of YAML whitespace, and part of it is because we rely on the parser to give configuration errors.

We should improve this experience by providing custom and informative errors.

Depends on: #771

[imsnif]

This was very much addressed as part of the KDL transfer. Our error messages now (especially for layouts) are top notch.