Fuseki responds 500 to some bad update requests

### Version

6.0.0

### What happened?

An update request with both `?using-named-graph-uri=` and `WITH` is illegal (SPARQL Protocol).

Fuseki catches this but mishandles the response, causing a 500, not 400.

### Relevant output and stacktrace

```shell

```

### Are you interested in making a pull request?

None