Deployment proceeds despite packaging failures

The `LambdaDeploymentPackager` prints a failure message when trying to build dependencies [here](https://github.com/aws/chalice/blob/master/chalice/deploy/packager.py#L88), but otherwise proceeds with deploying a (likely broken) Lambda function.

This is a bit cumbersome in a deployment toolchain as my deploy step currently succeeds and smoke-tests then fail. Is there a recommended way to have Chalice error out when packaging fails? I'm happy to send a PR for a `--strict` or `--abort-when-packaging-incomplete` packaging mode of some sort :)

See also #928.

Start in chalice/deploy/packager.py at the LambdaDeploymentPackager failure path around line 88, then trace how deployment handles that result. Review #928 for related context. Done means packaging failure causes the requested strict or abort behavior and the deploy step reports failure instead of proceeding.

Written by the indexing model from the issue text.