Add a link to explain about unmet exploit requirements

As an user, if you encounter an unmet exploit requirement, sometimes you don't necessarily know what it means, and it's impossible for us to explain everything in the console. So I decided create a wiki about it, and put a link in the ```print_warning```. The r-7.co URL is from bitly, which is created/managed under our account, so we can always be sure it won't be lost.

### Verification ###

- [ ] Make sure http://r-7.co/PVbcgx goes the the correct wiki that explains about unmet requirements.