Add more specific cause on web development tool error output (#98553)

packages/flutter_tools/lib/src/convert.dart

@@ -52,7 +52,7 @@ class Utf8Decoder extends cnv.Utf8Decoder {
     // was malformed.
     if (reportErrors && result.contains('\u{FFFD}')) {
       throwToolExit(
-        'Bad UTF-8 encoding found while decoding string: $result. '
+        'Bad UTF-8 encoding (U+FFFD; REPLACEMENT CHARACTER) found while decoding string: $result. '
         'The Flutter team would greatly appreciate if you could file a bug explaining '
         'exactly what you were doing when this happened:\n'
         'https://github.com/flutter/flutter/issues/new/choose\n'

packages/flutter_tools/test/general.shard/convert_test.dart

@@ -0,0 +1,41 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter_tools/src/base/common.dart';
+import 'package:flutter_tools/src/convert.dart';
+
+import '../src/common.dart';
+
+void main() {
+  late String passedString;
+  late String nonpassString;
+
+  const Utf8Decoder decoder = Utf8Decoder();
+
+  setUp(() {
+    passedString = 'normal string';
+    nonpassString = 'malformed string => <20>';
+  });
+
+  testWithoutContext('Decode a normal string', () async {
+    assert(passedString != null);
+
+    expect(decoder.convert(passedString.codeUnits), passedString);
+  });
+
+  testWithoutContext('Decode a malformed string', () async {
+    assert(nonpassString != null);
+
+    expect(
+      () => decoder.convert(nonpassString.codeUnits),
+      throwsA(
+        isA<ToolExit>().having(
+          (ToolExit error) => error.message,
+          'message',
+          contains('(U+FFFD; REPLACEMENT CHARACTER)'), // Added paragraph
+        ),
+      ),
+    );
+  });
+}