from the spec:
This member is REQUIRED.
It MUST be the same as the value of the id member in the Request Object.
If there was an error in detecting the id in the Request object (e.g. Parse error/Invalid Request), it MUST be Null.
```
data
A Primitive or Structured value that contains additional information about the error.
This may be omitted.
The value of this member is defined by the Server (e.g. detailed error information, nested errors etc.).
```
