add/unify i/o error handling

lots of use of Utils::FileSaver and Utils::FileReader

Task-number: QTCREATORBUG-1619

src/plugins/texteditor/snippets/snippetscollection.h

@@ -97,7 +97,7 @@ public:
     QList<QString> groupIds() const;
 
     void reload();
-    void synchronize();
+    bool synchronize(QString *errorString);
 
 private slots:
     void identifyGroups();