fix(cli): fix missing dockerfile error

[eablack]

Fixes #1420

Description

This resolves a bug where if you specify multiple process types in container:push --recursive and one of those dockerfiles doesn't actually exist, you get a bad error.

Testing

1. Pull down this branch and yarn build
2. Go to a directory where a docker-based app is set up
3. Have Dockerfile.web in the directory, but not Dockerfile.worker
4. run $CLI_PATH/bin/run container:push web worker --recursive
5. See warning message instead of error