release_config: add value_directories to artifact

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