lint: Error/warning messages should be instructive

With the linting functionality it is key that the error/warning messages we produce be as instructive to the user as possible. This means that the message should:

1. Explain why the error/warning is occuring
2. Explain what must be done to fix it
3. Give as much context as possible. So not "JSON array is empty", but instead "The `concepts` array in the config.json file must contain at least one concept`
4. Link to the documentation where the rule is defined