Update shader error message

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/PreviewManager.cs

                         {
                             var errors = MaterialGraphAsset.GetShaderErrors(m_UberShader);
                             var message = new ShaderStringBuilder();
-                            message.AppendLine("Uber shader has {0} error{1}:", errors.Length, errors.Length != 1 ? "s" : "");
+                            message.AppendLine(@"Preview shader for graph has {0} error{1}:", errors.Length, errors.Length != 1 ? "s" : "");
                             foreach (var error in errors)
                             {
                                 message.AppendLine("{0} at line {1} (on {2})", error.message, error.line, error.platform);