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.
Yup, we should update the other error messages as well.  Just talked to Troy and verified that it's not an error case to not have these scripts (with known names) in your project.  This type of distinction in logs (e.g. failure to start the script vs failure from the execution of the script) would be important while doing investigations.