I would say don't do it that way. Instead, do something like:
POST /requests
{
"id": 12,
"status": "Pending"
}
GET /requests/{id}=12
{
"id": 12,
"status": "Error",
"cause": "Invalid parameter: 'bob' is not a number"
}
GET /requests/{id}=15
{
"id": 15,
"status": "Complete",
"resource": "/resources/5"
}
GET /resources/5
{
"id": 5,
...
}
GET /requests
will return 200 OK
and indicate whether there's a problem or not. Clients can poll their request looking for "error" or "resource" (or however you want to structure it - I didn't think much about entities, only URIs).