Sets this's result to error (which this assumes
ownership of) and completes the task (see return_pointer for more discussion of
exactly what this means).
Note that since the task takes ownership of error, and since the task may be completed before returning from
return_error, you cannot assume that error is still valid after calling this. Call
copy on the error if you need to keep a local copy as well.
See also g_task_return_new_error.
| this |
a Task. |
| error |
the Error result of a task function. |