hub: improve inheritance priority collision error message

When users set multiple parents to the same priority in
setInheritanceData(), we raise an error because this is an invalid
configuration.

Improve the error message to make it easier for humans to understand the
problem and explain which tag ID ints are parents or children.

hub/kojihub.py

@@ -769,8 +769,9 @@ def _writeInheritanceData(tag_id, changes, clear=False):
             continue
         # oops, duplicate entries for a single priority
         dup_ids = [link['parent_id'] for link in dups]
-        raise koji.GenericError("Inheritance priorities must be unique (pri %s: %r )" %
-                                (pri, dup_ids))
+        raise koji.GenericError("Multiple parent tags (%r) cannot have the "
+                                "same priority value (%d) on child tag %d" %
+                                (dup_ids, pri, tag_id))
     for parent_id, link in data.items():
         if not link.get('is_update'):
             continue