Fix crash when querying backup mount status

src/backups.js

@@ -374,6 +374,8 @@ async function getMountStatus() {
         hostPath = backupConfig.mountPoint;
     } else if (backupConfig.provider === 'filesystem') {
         hostPath = backupConfig.backupFolder;
+    } else {
+        throw new BoxError(BoxError.BAD_STATE, 'Backup location is not a mount');
     }
 
     return await mounts.getStatus(backupConfig.provider, hostPath); // { state, message }