run-wrangler.js: send error to stderr and exit 1 if child process has an error

[jaredmcdonald]

fixes #848

ideally i'd add some tests but there doesn't seem to be a test suite for the npm shim. let me know if you have any preference there or if it's fine to just leave untested.

before:

> ./npm/run-wrangler.js --version
👷 ✨  wrangler 1.5.0
> mv ~/.wrangler/out/wrangler ~/.wrangler/out/wrangler.bak
> ./npm/run-wrangler.js --version
> echo $status
0

after:

> ./npm/run-wrangler.js --version
👷 ✨  wrangler 1.5.0
> mv ~/.wrangler/out/wrangler ~/.wrangler/out/wrangler.bak
jared@Jareds-MBP-2 ~/p/wrangler> ./npm/run-wrangler.js --version
{ Error: spawnSync /Users/jared/.wrangler/out/wrangler ENOENT
    at _errnoException (util.js:992:11)
    at spawnSync (child_process.js:579:20)
    at Object.<anonymous> (/Users/jared/projects/wrangler/npm/run-wrangler.js:16:16)
    at Module._compile (module.js:652:30)
    at Object.Module._extensions..js (module.js:663:10)
    at Module.load (module.js:565:32)
    at tryModuleLoad (module.js:505:12)
    at Function.Module._load (module.js:497:3)
    at Function.Module.runMain (module.js:693:10)
    at startup (bootstrap_node.js:191:16)
  code: 'ENOENT',
  errno: 'ENOENT',
  syscall: 'spawnSync /Users/jared/.wrangler/out/wrangler',
  path: '/Users/jared/.wrangler/out/wrangler',
  spawnargs: [ '--version' ] }
> echo $status
1

[EverlastingBugstopper]

Looks perfect! Not gonna make you start a test suite but feel free to open a separate PR we will not turn them away 😄