Watch e.g. the twitter registration tutorial video.
Pay attention to the steps, where they make sure, that the account got created.
You will do something similar:
If you can reproduce the error in the recorder: mark a part of the errormessage (e.g. 'username not available') right click, choose 'create presence check'
send these nodes to the editor and insert it into your template. Connect green to trying a different username, red to go on with the template.
In case you can't reproduce the error, you have to put the presence check in manually, err or not. Just do the same as above with a word that's on the page, and in the editor replace it with something from the errormessage.