Improve cdds_validate_streams error logging

Instead of specifying which stash codes are missing from a file it seems the set of all missing stash codes is printed for every problem file (which is quite misleading...).

Start by locating the cdds_validate_streams entry point and tracing how missing stash codes are collected and reported for each problem file. Reproduce the validation output with multiple problem files, then confirm that each file's error lists only its own missing stash codes.

Written by the indexing model from the issue text.