Hey Khris,
OutSystems should automatically generate the Response structure based on the JSON it receives. Maybe you should try consuming the REST API again or you can also add manually the structure based on the JSON. I also noticed the JSON you sent here is not completely valid, but I fixed the errors and generated a structure in OutSystems (there's an option if you right-click the Structures folder - Add structure from JSON) and it looks like this:
{
"data": [
{
"employeeData": [
{
"Error": "User Name is already exists"
}
]
}
],
"paging": null
}

If you don't already have it, you can try and manually add it to your response structure and see if it works. Then you should be able to get the error data from your structure.
Regards,
Bogdan