src: consolidate exit codes in the code base

Add an ExitCode enum class and use it throughout the code base instead of hard-coding the exit codes everywhere. At the moment, the exit codes used in many places do not actually conform to what the documentation describes. With the new enums (which are also available to the JS land as constants in an internal binding) we could migrate to a more consistent usage of the codes, and eventually expose the constants to the user land when they are stable enough.