Vague error message (Can't resolve '') when importing package with invalid entry point

**Summary**

Importing `react-hooks-firebase` causes snackager to return error:

```
curl "https://snackager.expo.io/bundle/react-firebase-hooks?version_snackager=true&platforms=ios,android,web"
Can't resolve '' in '/tmp/snackager/snackager/buildStatus/1/react-firebase-hooks@2.2.0-ios,android,web/package'
```

react-firebase-hooks does have a main entry in its package.json but that refers to a non existent file. The error message returned by Snackager is very vague

**What platform(s) does this occur on?**

Android, iOS, Web

**SDK Version**

40

**Reproducible demo or steps to reproduce from a blank project**

curl "https://snackager.expo.io/bundle/react-firebase-hooks?version_snackager=true&platforms=ios,android,web"