Exit code 0 when script fails to deploy

We had a mistake in our serverless function, but the exit code of `serverless deploy` was still 0, even though this plugin reported a "Fatal Error".

```
./node_modules/serverless/bin/serverless deploy
Serverless: Packaging service...
Serverless: Excluding development dependencies...
Serverless: Starting Serverless Cloudflare-Worker deployment.
Serverless: deploying route: https://my-route.example.com/*
Serverless: ❌ Fatal Error, Script Not Deployed!
Serverless: --> Error Code:10021
--> Error Message: "Uncaught ReferenceError: window is not defined
at line 16
at line 1 in r
at line 1
at line 1
"
Serverless: ✅ Routes Deployed
Serverless: Finished deployment in 6.789 seconds.
Serverless: Finished Serverless Cloudflare-Worker deployment.
exit code: 0 duration: 8s
```