show error notification when AppBackupWorker isn't successful

otherwise there's no UI feedback when running backup manually

app/src/main/java/com/stevesoltys/seedvault/worker/AppBackupWorker.kt

@@ -124,6 +124,8 @@ class AppBackupWorker(
                 Result.retry()
             } else {
                 val result = doBackup()
+                // show error notification if backup wasn't successful (maybe only when no retry?)
+                if (result != Result.success()) nm.onBackupError()
                 // only allow retrying if rescheduling is allowed
                 if (tags.contains(TAG_RESCHEDULE)) return result
                 else Result.success()