audiod: Properly handle wng open failures.

audiod: Properly handle wng open failures.

If opening the writer node group fails, no task is being registered to the
scheduler although a task structure is allocated. This makes audiod think
everything is fine until the output buffer of the receiver or a filter is
filled up which may take several seconds.

Fix this flaw by setting the error field of the wng to -E_TASK_UNREGISTERED.
This causes audiod to close the receiver node and the filter chain and to
clear the affected slot.