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.

… code base