Failed build returns 0 exit code

When `ember build` fails a non-zero error code should be returned. This is similar to #1150 

Steps to reproduce
```bash
ember init
# builds project and outputs OK
ember build && echo "OK"
# remove some dependency to break the build
rm -rf vendor/jquery
ember build && echo "OK"
```

Output:
```bash
version: 0.0.36
Build failed.
Path or pattern "vendor/jquery/dist/jquery.js" did not match any files
Error: Path or pattern "vendor/jquery/dist/jquery.js" did not match any files
    at Object.multiGlob (./node_modules/ember-cli/node_modules/broccoli-concat/node_modules/broccoli-kitchen-sink-helpers/index.js:216:13)
    at ./node_modules/ember-cli/node_modules/broccoli-concat/index.js:41:30
    at tryCatch (./node_modules/ember-cli/node_modules/rsvp/dist/commonjs/rsvp/-internal.js:163:16)
    at invokeCallback (./node_modules/ember-cli/node_modules/rsvp/dist/commonjs/rsvp/-internal.js:172:17)
    at publish (./node_modules/ember-cli/node_modules/rsvp/dist/commonjs/rsvp/-internal.js:150:13)
    at flush (./node_modules/ember-cli/node_modules/rsvp/dist/commonjs/rsvp/asap.js:51:9)
    at process._tickCallback (node.js:419:13)OK
```