check if curl is on the path

Hi @jasbro,

This pull request references https://github.com/rapid7/metasploit-framework/pull/1596 , it tries to check if curl is on the path before proceeding. After testing, I think it's valuable to provide this information to the user, because when curl isn't available on the $PATH it just exists silently. What do you think? Feel free to review, and if you agree with the changes feel free to land this pull request to your repo and the original one will be updated automatically :)