+ /** These strings should say what is wrong, incorporating any extra details (ID, filenames etc.).
+ *
+ * e.g. "ClosedCaption asset has no <EntryPoint> tag.",
+ * not "ClosedCaption assets must have an <EntryPoint> tag."
+ *
+ * It's OK to use XML tag names where they are clear.
+ * If both ID and filename are available, use only the ID.
+ * End messages with a full stop.
+ * Messages should not mention whether or not their errors are a part of Bv2.1.
+ */