logic: allow the user to discard an aborted indexer run

* keep old database on filesystem while indexing
* migrate old ".coatiproject" settings to new ".srctrlprj" extension when loading project
* send new errors in MessageErrorCountUpdate after injecting
* recheck filepath exist() in FileSystem's remove, rename, copy and create functions
* updated tasks to new "override" policy

src/lib_gui/qt/window/QtIndexingDialog.cpp

@@ -207,8 +207,16 @@ void QtIndexingDialog::setupReport(
 	layout->addStretch();
 
 	addButtons(layout);
-	updateNextButton("OK");
-	setCloseVisible(false);
+	if (interrupted)
+	{
+		updateNextButton("Keep");
+		updateCloseButton("Discard");
+	}
+	else
+	{
+		updateNextButton("OK");
+		setCloseVisible(false);
+	}
 
 	m_sizeHint = QSize(interrupted ? 400 : 430, 280);