Always validate Backup before restoring (#5632)

* Validate Backup always before restoring

Since #5519 we check the encryption password early in restore case.
This has the side effect that we check the file existance early too.
However, in the non-encryption case, the file is not checked early.

This PR changes the behavior to always validate the backup file before
restoring, ensuring both encryption and non-encryption cases are
handled consistently.

In particular, the last case of test_restore_immediate_errors actually
validates that behavior. That test should actually have failed so far.
But it seems that because we validate the backup shortly after freeze
anyways, the exception still got raised early enough.

A simply `await asyncio.sleep(10)` right after the freeze makes the
test case fail. With this change, the test works consistently.

* Address pylint

* Fix backup_manager tests

* Drop warning message

tests/backups/test_manager.py

@@ -344,7 +344,7 @@ async def test_fail_invalid_full_backup(
 
     backup_instance = full_backup_mock.return_value
     backup_instance.all_locations[None]["protected"] = True
-    backup_instance.validate_password = AsyncMock(return_value=False)
+    backup_instance.validate_backup.side_effect = BackupInvalidError()
 
     with pytest.raises(BackupInvalidError):
         await manager.do_restore_full(backup_instance)
@@ -373,7 +373,7 @@ async def test_fail_invalid_partial_backup(
 
     backup_instance = partial_backup_mock.return_value
     backup_instance.all_locations[None]["protected"] = True
-    backup_instance.validate_password = AsyncMock(return_value=False)
+    backup_instance.validate_backup.side_effect = BackupInvalidError()
 
     with pytest.raises(BackupInvalidError):
         await manager.do_restore_partial(backup_instance)