chore(shorebird_cli): improve error message when Flutter project does not support macOS (#2777)

packages/shorebird_cli/lib/src/validators/macos_network_entitlement_validator.dart

@@ -48,8 +48,13 @@ class MacosNetworkEntitlementValidator extends Validator {
   String get description => 'macOS app has Outgoing Connections entitlement';
 
   @override
-  bool canRunInCurrentContext() =>
-      _macosDirectory != null && _macosDirectory!.existsSync();
+  bool canRunInCurrentContext() => _macosDirectory?.existsSync() ?? false;
+
+  @override
+  String? get incorrectContextMessage => '''
+The ${_macosDirectory?.path ?? 'macos'} directory does not exist.
+
+The command you are running must be run within a Flutter app project that supports the macOS platform.''';
 
   @override
   Future<List<ValidationIssue>> validate() async {

packages/shorebird_cli/test/src/validators/macos_network_entitlement_validator_test.dart

@@ -100,6 +100,12 @@ void main() {
             runWithOverrides(() => validator.canRunInCurrentContext()),
             isFalse,
           );
+          expect(
+            runWithOverrides(() => validator.incorrectContextMessage),
+            contains(
+              '''The command you are running must be run within a Flutter app project that supports the macOS platform.''',
+            ),
+          );
         });
       });
     });