确认下载路径之前检查盘符是否存在

src/DownKyi/Languages/Default.xaml

@@ -345,6 +345,7 @@
     <system:String x:Key="IsDefaultDownloadDirectoryTip">选中后下次将不会弹出此窗口</system:String>
     <system:String x:Key="Download">下载</system:String>
     <system:String x:Key="WarningNullDirectory">文件夹路径不能为空</system:String>
+    <system:String x:Key="DriveNotFound">该磁盘不存在</system:String>
 
     <!--  ViewParsingSelector  -->
     <system:String x:Key="ParsingSelector">选择解析项</system:String>

src/DownKyi/Services/Download/AddToDownloadService.cs

@@ -191,6 +191,14 @@ namespace DownKyi.Services.Download
                 });
             }
 
+            if (!Directory.Exists(Directory.GetDirectoryRoot(directory)))
+            {
+                var alert = new AlertService(dialogService);
+                alert.ShowError(DictionaryResource.GetString("DriveNotFound"));
+
+                directory = string.Empty;
+            }
+
             // 下载设置dialog中如果点击取消或者关闭窗口，
             // 会返回空字符串，
             // 这时直接退出