data_ingestion/jmdict: throw proper errors on invalid xrefs

lib/_data_ingestion/jmdict/seed_data.dart

@@ -27,15 +27,44 @@ ResolvedXref resolveXref(
   SplayTreeMap<String, Set<Entry>> entriesByReading,
   XRefParts xref,
 ) {
-  List<Entry> candidateEntries = switch ((xref.kanjiRef, xref.readingRef)) {
-    (null, null) => throw Exception(
-      'Xref $xref has no kanji or reading reference',
-    ),
-    (final String k, null) => entriesByKanji[k]!.toList(),
-    (null, final String r) => entriesByReading[r]!.toList(),
-    (final String k, final String r) =>
-      entriesByKanji[k]!.intersection(entriesByReading[r]!).toList(),
-  };
+  late List<Entry> candidateEntries;
+  switch ((xref.kanjiRef, xref.readingRef)) {
+    case (null, null):
+      throw Exception('Xref $xref has no kanji or reading reference');
+
+    case (final String k, null):
+      if (!entriesByKanji.containsKey(k)) {
+        throw Exception(
+          'Xref $xref has kanji reference "$k" but no entries found with that kanji',
+        );
+      }
+      candidateEntries = entriesByKanji[k]!.toList();
+      break;
+
+    case (null, final String r):
+      if (!entriesByReading.containsKey(r)) {
+        throw Exception(
+          'Xref $xref has reading reference "$r" but no entries found with that reading',
+        );
+      }
+      candidateEntries = entriesByReading[r]!.toList();
+      break;
+
+    case (final String k, final String r):
+      if (!entriesByKanji.containsKey(k)) {
+        throw Exception(
+          'Xref $xref has kanji reference "$k" but no entries found with that kanji',
+        );
+      }
+      if (!entriesByReading.containsKey(r)) {
+        throw Exception(
+          'Xref $xref has reading reference "$r" but no entries found with that reading',
+        );
+      }
+      candidateEntries = entriesByKanji[k]!
+          .intersection(entriesByReading[r]!)
+          .toList();
+  }
 
   // Filter out entries that don't have the number of senses specified in the xref
   if (xref.senseOrderNum != null) {