Error handling according to fixed set of relations for play-spoken-text

[hsluytergaethje]

In the endpoint /corpora/{corpusname}/plays/{playname}/spoken-text for the parameter relation a fixed set of possible values is indicated in the API documentation. When I pass a parameter that is not in the list an empty string is returned. It would be good to get an error (similar to the gender parameter) in which the possible set of values is indicated.
The same would be good for the parameter role or is this not a fixed set of values?