Each task in a Maintenance Plan has a variety of options that can be changed by right-clicking the desired task, then selecting the "Properties" option from the context-menu.  As in:

[![enter image description here][1]][1]

To allow a task to have multiple errors prior to failing, simply modify the `MaximumErrorCount` property I've highlighted above to some number greater than 1.

Having said that, Maintenance Plans are difficult to manage.  

Options I recommend using instead:

- [Ola Hallengren](https://ola.hallengren.com) has a great replacement solution that is extensible and makes management a breeze.  

- [MinionWare Backup](http://www.minionware.net/products/backup/) - built by DBAs for DBAs.

  [1]: https://i.sstatic.net/iXrTZ.png