Output error if `keymaps` dir is missing

[waffle87]

Description

As title. Otherwise, build will just fail silently. Let me know if there's somewhere else this should go — feel like there's numerous places/methods for this.

Types of Changes

• [x] Core
• [x] Enhancement/optimization

Checklist

• [x] My code follows the code style of this project: C, Python
• [x] I have read the PR Checklist document and have made the appropriate changes.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [x] I have tested the changes and verified that they work and don't break anything (as well as I can manage).