process: Fix regression from a5dba82

Respect numeric string args to process.exit(code)
process: use exit 1 for uncaughtException

Since it is Unix tradition to use exit code 1 for general-purpose script
bail-out, and the way of doing that in Node is to throw an exception and
not catch it, it makes the most sense to exit with 1 when an exception
goes uncaught.

Move the `Invalid Argument` exit to 9, so that it's something specific,
and clear that it's a node internal error.

Also, document the exit codes that we use.