Return the exit code in ScriptExecutor

Solving issue #987

Now the exit code from the script in project.json need to be zero to ensure a successful build/pack/restore.
I'll update the the messages. How about "The exit status of the prepare script is not zero."? 

In terms of resource strings, I sample a dozen of error messages in PackageManager none of them is using resources. And we do need to start using string resource. However, can we get over this change first and scrub the PackageManager with string resources in another PR?