GODT-1381: Use in-memory cache in case local cache is unavailable

- change: refactor GUI notification object
- add: global bridge errors
- add: when cache on disk cannot be initialized fallback to memory cache
- add: show notification for CoD failure
- change: do not allow login to IMAP and SMTP when CoD init failed

internal/frontend/qml/Bridge.qml

@@ -52,6 +52,13 @@ QtObject {
         onVisibleChanged: {
             backend.dockIconVisible = visible
         }
+
+        Connections {
+            target: root.backend
+            onCacheUnavailable: {
+                mainWindow.showAndRise()
+            }
+        }
     }
 
     property StatusWindow _statusWindow: StatusWindow {