Improve error message when not being run from a track's root directory

We should check whether the tool is being run from the right directory, and if not, output a nice error message. The things we can check for are:

- `config.json` file in the current working directory
- `exercises` directory exists if there are exercises specified in the `config.json` file
- `exercises/` directory exists for each exercise in the `config.json` file