stylelint: report errors in the exitCode

[evannoronha]

Without this flag, it's hard to determine programmatically (i.e. in CI) that our code didn't pass the linter.

What kind of change does this PR introduce? (check at least one)

• [ ] Bugfix
• [x] Feature
• [ ] Code style update
• [ ] Refactor
• [ ] Docs
• [ ] Other, please describe:

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [x] No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• [x] It's submitted to the dev branch, not the master branch

Other information: We're currently using packer lint in CI, and the command is reporting success even though there are style violations.

We can hack around this by inspecting console output, but this approach is cleaner.