Would it make sense to return a 204 (from http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html) if it is still running.

204 No Content

The server has fulfilled the request but does not need to return an entity-body, and might want to return updated metainformation. The response MAY include new or updated metainformation in the form of entity-headers, which if present SHOULD be associated with the requested variant.

If we detect something wrong we can return a 404.