Is there a way to make Serverless abort execution if the stage is not given?

Looks like it defaults to "dev", which is not preferred in a multi-environment setup where a "default" environment doesn't exist.

The closest I could get was reading a 'stage' config variable from a local file. Unfortunately Serverless still defaults to 'dev' if the stage variable is missing from the (existing) local file. It does give a warning for the missing variable, though.

```
provider:
  name: aws
  runtime: nodejs8.10
  region: eu-west-1
  stage: ${file(serverless-local.yml):stage}

```