cloudize-workflow.py should fail if it encounters an invalid file path or access issue

[malachig]

Currently if the cloudize script encounters a file path in the input "local" YAML, and it can not access that file, it just silently skips over it, prints out an unmodified YAML record for that line, and declares victory.

We probably want it to instead check whether file paths are valid and error out or at least warn.