In the SetQualifier module, I gave it invalid data value input (and it was being handled incorrectly in the api). I got back an exception with full stack trace in the api result.
Generally speaking, in a number of places, the api is also doing insufficient validation and error handling of the data values. The api user should *not* see an exception in the api error output. Any exceptions due to invalid value input should be caught and translated into a proper api error message and code.
(See also related bug 48864 and bug 48473)