Raise an exception when invalid subshard name (#113222)

dev/bots/test.dart

@@ -1972,7 +1972,7 @@ List<T> _selectIndexOfTotalSubshard<T>(List<T> tests, {String subshardKey = kSub
     foundError(<String>[
       '${red}Invalid subshard name "$subshardName". Expected format "[int]_[int]" ex. "1_3"',
     ]);
-    return <T>[];
+    throw Exception('Invalid subshard name: $subshardName');
   }
   // One-indexed.
   final int index = int.parse(match.group(1)!);

dev/bots/test/test_test.dart

@@ -144,5 +144,13 @@ void main() {
       expectExitCode(result, 1);
       expect(result.stdout, contains('Invalid subshard name'));
     });
+
+    test('exits with code 255 when invalid SUBSHARD name', () async {
+      final ProcessResult result = await runScript(
+        <String, String>{'SHARD': kTestHarnessShardName, 'SUBSHARD': 'invalid_name'},
+      );
+      expectExitCode(result, 255);
+      expect(result.stdout, contains('Invalid subshard name'));
+    });
   });
 }