fix(shorebird_cli): Print stderr output when flutter version check fails (#270)

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

@@ -2,6 +2,13 @@ import 'package:shorebird_cli/src/shorebird_environment.dart';
 import 'package:shorebird_cli/src/shorebird_process.dart';
 import 'package:shorebird_cli/src/validators/validators.dart';
 
+class FlutterValidationException implements Exception {
+  const FlutterValidationException(this.message);
+
+  /// The message associated with the exception.
+  final String message;
+}
+
 class ShorebirdFlutterValidator extends Validator {
   ShorebirdFlutterValidator({required this.runProcess});
 
@@ -120,11 +127,21 @@ This can cause unexpected behavior if the version gap is wide. If you're seeing
       ['--version'],
       useVendedFlutter: !checkPathFlutter,
     );
-    final output = result.stdout.toString();
 
+    if (result.exitCode != 0) {
+      throw FlutterValidationException(
+        '''
+        Flutter version check did not complete successfully.
+        ${result.stderr}''',
+      );
+    }
+
+    final output = result.stdout.toString();
     final match = _flutterVersionRegex.firstMatch(output);
     if (match == null) {
-      throw Exception('Could not find version match in $output');
+      throw FlutterValidationException(
+        'Could not find version match in $output',
+      );
     }
 
     return match.group(1)!;

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

@@ -103,8 +103,12 @@ Tools • Dart 2.19.6 • DevTools 2.20.1
 
       when(() => pathFlutterVersionProcessResult.stdout)
           .thenReturn(pathFlutterVersionMessage);
+      when(() => pathFlutterVersionProcessResult.stderr).thenReturn('');
+      when(() => pathFlutterVersionProcessResult.exitCode).thenReturn(0);
       when(() => shorebirdFlutterVersionProcessResult.stdout)
           .thenReturn(shorebirdFlutterVersionMessage);
+      when(() => shorebirdFlutterVersionProcessResult.stderr).thenReturn('');
+      when(() => shorebirdFlutterVersionProcessResult.exitCode).thenReturn(0);
       when(() => gitBranchProcessResult.stdout).thenReturn(gitBranchMessage);
       when(() => gitStatusProcessResult.stdout).thenReturn(gitStatusMessage);
     });
@@ -196,5 +200,22 @@ Tools • Dart 2.19.6 • DevTools 2.20.1
 
       expect(() async => validator.validate(), throwsException);
     });
+
+    test('prints stderr output and throws if version check fails', () async {
+      when(() => pathFlutterVersionProcessResult.exitCode).thenReturn(1);
+      when(() => pathFlutterVersionProcessResult.stderr)
+          .thenReturn('error getting Flutter version');
+
+      expect(
+        () async => validator.validate(),
+        throwsA(
+          isA<FlutterValidationException>().having(
+            (e) => e.message,
+            'message',
+            contains('error getting Flutter version'),
+          ),
+        ),
+      );
+    });
   });
 }