Changed read_non_inherited_metadata to raise exception on issue

* Previously the read_non_inherited_metadata behavior would try to
  open a file and, if the file was not readable or not found, would
  push a debug message and return an empty list. This leads to tests
  that appear to fail because an inherited metadata key/value pair
  is not found in the child's metadata. These failures can be red
  herring if the issue is actually that the file containing keys
  that should not be inherited is not found/readable.
* This change should raise an exception and error the test if a
  file path is passed to the non_inherited_metadata_filepath value
  in the config and that file cannot be opened.

Change-Id: I0292f3e28b91f59fb290f3ec85a93d1515c4eb05
Merge "Changed read_non_inherited_metadata to raise exception on issue"