fix codra-server: returns previous parse when parsing 'illegal input'

[arne-cl]

How to reproduce:

• parse some "normal text", e.g.

Students and lecturers alike are worried about global warming and related climate change.
However, their own contribution to climate change is all too often ignored.

• parse something that codra can't handle, e.g. foo
• rst-workbench will now show the parse tree of the Students and lecturers text.