feat(content-build): scaffold package + Markdown yaml-block parser

tools/content_build/errors.py

@@ -0,0 +1,12 @@
+"""The single failure type. Every validation/parse problem raises BuildError so
+the CLI can print a uniform 'BUILD FAILED: <what> [<id>] (<file:line>)'."""
+
+
+class BuildError(Exception):
+    def __init__(self, message: str, source: str = "", entry_id: str = ""):
+        self.message = message
+        self.source = source
+        self.entry_id = entry_id
+        loc = f" [{entry_id}]" if entry_id else ""
+        src = f" ({source})" if source else ""
+        super().__init__(f"{message}{loc}{src}")