Improve arb FormatException error message (#56373)

dev/tools/localization/gen_l10n_types.dart

@@ -363,8 +363,16 @@ class AppResourceBundle {
   factory AppResourceBundle(File file) {
     assert(file != null);
     // Assuming that the caller has verified that the file exists and is readable.
+    Map<String, dynamic> resources;
+    try {
+      resources = json.decode(file.readAsStringSync()) as Map<String, dynamic>;
+    } on FormatException catch (e) {
+      throw L10nException(
+        'The arb file ${file.path} has the following formatting issue: \n'
+        '${e.toString()}',
+      );
+    }
 
-    final Map<String, dynamic> resources = json.decode(file.readAsStringSync()) as Map<String, dynamic>;
     String localeString = resources['@@locale'] as String;
     if (localeString == null) {
       final RegExp filenameRE = RegExp(r'^[^_]*_(\w+)\.arb$');

dev/tools/test/localization/gen_l10n_test.dart

@@ -1284,7 +1284,10 @@ import 'output-localization-file_en.dart' deferred as output-localization-file_e
       });
     });
 
-    test('should throw when failing to parse the arb file', () {
+    test(
+      'should throw with descriptive error message when failing to parse the '
+      'arb file',
+      () {
         const String arbFileWithTrailingComma = '''
 {
   "title": "Stocks",
@@ -1308,16 +1311,19 @@ import 'output-localization-file_en.dart' deferred as output-localization-file_e
           );
           generator.loadResources();
           generator.writeOutputFiles();
-      } on FormatException catch (e) {
+        } on L10nException catch (e) {
+          expect(e.message, contains('app_en.arb'));
+          expect(e.message, contains('FormatException'));
           expect(e.message, contains('Unexpected character'));
           return;
         }
 
         fail(
-        'should fail with a FormatException due to a trailing comma in the '
+          'should fail with an L10nException due to a trailing comma in the '
           'arb file.'
         );
-    });
+      },
+    );
 
     test('should throw when resource is missing resource attribute', () {
       const String arbFileWithMissingResourceAttribute = '''