The ReplacementTransformer already checks for the error condition where
a replacement source doesn't match any of the input documents.
This change adds similar error handling for missing replacement
target documents, and tweaks the naming/messages to make it clear
which situation is encountered.
This will make it easier for manifest engineers to catch typos
in their RT rules, or documents that have failed to be kustomized in
or authored.
Change-Id: I0efbcfbc2952afae05e48b9ff8e0b3c67df7e7df
2e7224a7bf