feature request: Break the build when critical YAML frontmatter is not valid

[joewrightfd]

During the build you get some helpful error messages to tell you when your data is not right like:

Item behaviour-driven-development has invalid quadrant adopt

In this case someone has flipped the quadrant and rings around. Currently, the build passes when this is encountered. It'd be good to optionally choose to break the build at this point.

Additional: if you know the valid quadrants, you could list them in the error message to help folks know what is wrong.

So for our example (colour would help with folks reading this):

Item behaviour-driven-development has invalid quadrant adopt
  valid quadrants are: languages-and-frameworks, methods-and-patterns, platforms-and-operations, tools

Thanks for this wonderful project.