Make the error for missing IDs that MDN expects less confusing.

bikeshed/mdn/mdnspeclinks.py

@@ -91,14 +91,15 @@ def panelsFromData(doc: t.SpecT, data: MdnDataT) -> list[t.ElementT]:
     }
 
     panels = []
+    missingIds = []
     for elementId, features in data.items():
         lessThanTwoEngines = 0
         onlyTwoEngines = 0
         allEngines = 0
         featureDivs = []
         targetElement = h.find(f"[id='{elementId}']", doc)
         if targetElement is None and elementId not in doc.md.ignoreMDNFailure:
-            m.warn(f"No '{elementId}' ID found, skipping MDN features that would target it.")
+            missingIds.append(elementId)
             continue
 
         for feature in features:
@@ -150,6 +151,11 @@ def panelsFromData(doc: t.SpecT, data: MdnDataT) -> list[t.ElementT]:
         panels.append(anno)
         h.appendChild(doc.body, anno)
 
+    if missingIds:
+        msg = "Skipped generating some MDN panels, because the following IDs weren't present in the document:\n"
+        msg += "\n".join("  #"+missingId for missingId in missingIds)
+        m.warn(msg)
+
     return panels