Why did completeTasks error?

Question

I am creating a webApi to trigger a!completeTask(). If this encounters an error, I return an httpResponse to indicate such. 
 The question is: how do I know what the error was? Is there some way to return the task completion error in the webApi? 
 
 Thank you. 
 
 a!completeTask(
 taskId: ri!taskId,
 taskInputs: {
 someData: "abc",
 moreData: "def"
 },
 onSuccess: a!httpResponse(200, {}, "Success")
 onError: a!httpResponse(500, {}, "What was the error??")
 )

Shubham Aware · Answer

Smart services do not expose internal system error details directly in the onError. Custom error messages must be handled by performing validations before calling the smart service. If validation fails, you can return specific error details. You have to rely on logs.

Harsha Sharma · Answer

You can work with the data the httpResponse sends by matching it with a user friendly message to show to user. 
 For detailed errors there is no way to show it directly using the response as that is available within log files.