lint: Error/warning messages should be instructive

[ErikSchierboom]

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