Editor manager: Abort with a single message if file is not readable.

We show a dialog that offers opening a file in a different editor type
if opening a file fails, but we should not do that if opening the file
fails because it is not readable.
With this change, documents now specify if they failed to open a file
because reading failed, or because they could not handle the file
contents.

Task-number: QTCREATORBUG-14495
Change-Id: I5d4b7cfa74b87ef21b9b55bc30b3ebe2f8238dfa
Reviewed-by: Daniel Teske <daniel.teske@theqtcompany.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>

src/plugins/tasklist/tasklistplugin.cpp

@@ -177,7 +177,7 @@ IDocument *TaskListPlugin::openTasks(const QString &base, const QString &fileNam
     file->setBaseDir(base);
 
     QString errorString;
-    if (!file->open(&errorString, fileName, fileName)) {
+    if (!file->load(&errorString, fileName)) {
         QMessageBox::critical(ICore::mainWindow(), tr("File Error"), errorString);
         delete file;
         return 0;