Return different exit code depending on success

[marco-calautti]

Currently, regardless if the exploit fails or succedees, the executable returns 0 as exit code. It would be useful to return a non-zero exit code in case of failure.

[xfangfang]

Thanks for the suggestion, done by: https://github.com/xfangfang/PPPwn_cpp/commit/89158dc9eaf9c158d27ebda1c141882b3386d7f0