Refactor julia_init

Specifically:

1. Refactor code to be more readable;
2. Add comments;
3. Improve syntax of the error message if Julia cannot be found;
4. Move that check to the if-check, allowing us to avoid the else{} control flow (in the event of a stop(), nothing further is executed in that scope, making a full if/else extraneous if the stop() is the "if");