add errorString output argument to IFile::reload() & IEditor::open()

add/unify read error handling in all affected classes

src/plugins/genericprojectmanager/genericproject.h

@@ -172,7 +172,7 @@ public:
     virtual void rename(const QString &newName);
 
     ReloadBehavior reloadBehavior(ChangeTrigger state, ChangeType type) const;
-    void reload(ReloadFlag flag, ChangeType type);
+    bool reload(QString *errorString, ReloadFlag flag, ChangeType type);
 
 private:
     GenericProject *m_project;