Tricky behavior around build zip filenames

Because of the [logic that fastboot-s3-downloader uses](https://github.com/tomdale/fastboot-s3-downloader/blob/master/index.js#L128) to extract output path from zip file names, any .zip filename without a dash in it will cause fastboot-aws to proceed with an outputPath of `""` ultimately resulting in rimraf crashing when it tries to delete a path of `""`.

The error message the user gets is fairly cryptic (pretty sure this is the exact quote: `something something rimraf became unhappy`)

I suggest a few courses of action
- in the instructions for fastboot-aws on ebs setup, mention that the filename should be something fairly consistent with the expected pattern (i.e., `dist-0.zip`)
- add a few asserts along the way -- particularly considering that once you have your asset serving layer running on EBS, pulling the logs is not effortless, and in fact cannot be done while an environment is busy doing anything else at all.