logic: Fixed crashes and halting during indexing

* Fixed CPPSqliteException being thrown due to not properly destructed statement in addError
* Fixed indexer getting stuck after building caches because MessageFinishedParsing was interrupted
* removed Message::cancel() method and all related code

bug id = 105

src/test/TaskSchedulerTestSuite.h

@@ -32,7 +32,7 @@ public:
 		int order = 0;
 		TestTask task(&order, 1);
 
-		task.execute();
+		task.executeTask();
 
 		TS_ASSERT_EQUALS(3, order);