fix(storage): refuse to open a workspace whose database file is missing

With .ykanban present but ykanban.db gone, opening used to create an empty
database and show an empty board without any hint. It now throws
WorkspaceDatabaseMissingException and shows the open-failed page, telling the
user to restore a backup or delete .ykanban and initialize again.

YKanBan.Tests/ViewModels/OpenFailedPageViewModelTests.cs

@@ -43,6 +43,9 @@ public class OpenFailedPageViewModelTests
             Assert.AreEqual(
                 Expected(Resources.Error_OpenFailed_SchemaTooNew, 3L, 1),
                 OpenFailedPageViewModel.BuildMessage(Folder, new SchemaVersionException(3, 1)));
+            Assert.AreEqual(
+                Expected(Resources.Error_OpenFailed_DatabaseMissing),
+                OpenFailedPageViewModel.BuildMessage(Folder, new WorkspaceDatabaseMissingException("/work/repo/.ykanban/ykanban.db")));
             Assert.AreEqual(
                 Expected(Resources.Error_OpenFailed_Database),
                 OpenFailedPageViewModel.BuildMessage(Folder, new SqliteException("file is not a database", 26)));