TaskTree: Get rid of onGroupError element

Make it possible to setup onGroupDone element with additional
OnDone argument.
The onGroupDone handler may accept extra DoneResult argument.
The onGroupDone handler may also tweak the success bit of a group.

All above features conform to the behavior of the task done handler.

Task-number: QTCREATORBUG-29834
Change-Id: I125bdfe155e585678fb33410632246401cbc9390
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: hjk <hjk@qt.io>

src/plugins/git/branchmodel.cpp

@@ -468,8 +468,7 @@ void BranchModel::refresh(const FilePath &workingDirectory, ShowError showError)
     const Group root {
         topRevisionProc,
         ProcessTask(onForEachRefSetup, onForEachRefDone),
-        onGroupDone(finalize),
-        onGroupError(finalize)
+        onGroupDone(finalize)
     };
     d->refreshTask.reset(new TaskTree(root));
     d->refreshTask->start();