Plugin does not handle situation when there is no change

It would be good if you check in s3 first if the object exists.

```
> sls deploy --changeset --stage dev

Serverless: Packaging service...
Serverless: Service files not changed. Skipping deployment...
Serverless: Creating CloudFormation ChangeSet [...]...

  Serverless Error ---------------------------------------

  S3 error: Unable to get the object https://s3.amazonaws.com/...
```