release_config_artifact.directories contains the ordered list of
release config map directories that contributed flag declarations and/or
flag values to the release config (ignoring inheritance).
release_config_artifact.value_directories contains the ordered list of
release config map directories where we found a release_config message
for this release config.
Also, improve various error messages.
Bug: None
Test: manual, TH
Change-Id: Ifb67e80fc8746ae466f2e3515c5b0c3ba07a291d
9877b7e627