Added clearer error when missing env vars for aws credentials.

Before this change the error looks like.
```
$ node awsbox.js zones
fatal error: error running 'zones' command: secretAccessKey and accessKeyId must be set
```

After change.

```
fatal error: Missing aws credentials
Please configure the AWS_ID and AWS_KEY environment variables.
```

Makes it a little clearer to the user what they need to correct :smile: and does it before entering any other code.

Cheers