A command that fails on a layer id or name the document doesn't have now says which layers there are, top-down as id and name (at most 20, names cut at 64 characters), so an agent can recover from the error alone: ids are not positions, and a freshly opened PNG's Background is layer 2. NoLayer becomes UnknownLayer { id, known } at both command entry points, keeping the 'no such layer' prefix. A layer name that matches no layer exactly matches regardless of case; an exact match always wins.
Fixes#5058.
