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.
The benefit of extract the value to a const is to make it easier to change expected successful exit code universally. However using `0` as a successful exit code is conventional in most situation, I don't see this coming. On the other hand leave the `0` in the expression make the codes more readable.