Fixed Debian bug #1073575 - nala returns zero when the user does not want to continue

There's a bug tracked under #1073575 where nala returns an exit code of zero (0) when an install is aborted by pressing N when prompted.

After reproducing it on my local machine with latest Debian Sid, I went ahead, searched for the prompt "Do you want to continue?" string in the source code and edited the sys.exit(). Built, checked it's fixed, then commit, push, merge requested.

Can't believe one bit, just one single bit was causing it! (No offence to you devs though, I can very well understand how simple it is to make such an error. It happens to everyone.)

I ask you to push this change to the package repos at earliest convenience, please.