You could place the entire implementation of your published web service operation in another microflow. Then you could call that microflow using a microflow call activity with custom error handling to "catch" any system errors and then throw a custom error (e.g. using a Java action).
The default parameter error messages (such as errors for required values and wrong value types) are currently not configurable. However, you could always perform additional validation on the parameters in your microflow and throw custom errors for those.