fix: do not overwrite user-defined process's exit code to 1

[micalevisk]

Instead of exiting with 1 we must keep the user-land value defined for process.exitCode if it's non-zero otherwise we end up changing the error code defined by the consumer of this lib.

Code example:

// app.js
require('source-map-support').install();
process.exitCode = 123
throw new Error('test 123')

after running node app.js I'm expecting to see 123 as the exit code but I'm getting 1: