Implement proper return/exit code if bundle process fails

[erfanimani]

I.e.

/var/www/html $ bundle
ℹ Using bundling config from "/var/www/html/magepack.config.js".                                                                                                                                                03:49:49

 ERROR  Cannot find module '/var/www/html/magepack.config.js'                                                                                                                                                   03:49:49
Require stack:
- /usr/lib/node_modules/magepack/lib/bundle.js
- /usr/lib/node_modules/magepack/cli.js

  Require stack:
  - /usr/lib/node_modules/magepack/lib/bundle.js
  - /usr/lib/node_modules/magepack/cli.js
  at Function.Module._resolveFilename (internal/modules/cjs/loader.js:966:15)
  at Function.Module._load (internal/modules/cjs/loader.js:842:27)
  at Module.require (internal/modules/cjs/loader.js:1026:19)
  at require (internal/modules/cjs/helpers.js:72:18)
  at module.exports (/usr/lib/node_modules/magepack/lib/bundle.js:18:28)
  at Command.<anonymous> (/usr/lib/node_modules/magepack/cli.js:47:32)
  at Command.listener [as _actionHandler] (/usr/lib/node_modules/magepack/node_modules/commander/index.js:413:31)
  at Command._parseCommand (/usr/lib/node_modules/magepack/node_modules/commander/index.js:914:14)
  at Command._dispatchSubcommand (/usr/lib/node_modules/magepack/node_modules/commander/index.js:865:18)
  at Command._parseCommand (/usr/lib/node_modules/magepack/node_modules/commander/index.js:882:12)

/var/www/html $ echo $?
0

Expected return code: 1

Actual return code: 0

This would be very useful for CI/CD. At the moment if an error occurs, the job has no way of knowing whether the process was successful or not.