relations-csv returns 404 for both play not found and relations not present

[hsluytergaethje]

The endpoint /corpora/{corpusname}/plays/{playname}/relations/csv returns 404 when the play is not found as well as when no relations exist. Would it be possible to make the error more specific?

[cmil]

Maybe it would be less ambiguous to return 404 only when the play does not exist, but return kind of empty documents when the play is missing relation. That would mean for the CSV we would only return the header line, and for the XML formats an XML document without any node and edge elements. What do you think @lehkost and @ingoboerner.