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/resourceeditor/resourcenode.cpp

@@ -139,7 +139,7 @@ void ResourceTopLevelNode::update()
     QMap<QPair<QString, QString>, QList<ProjectExplorer::FileNode *> > filesToAdd;
 
     ResourceFile file(path().toString());
-    if (file.load()) {
+    if (file.load() == Core::IDocument::OpenResult::Success) {
         QSet<QPair<QString, QString > > prefixes;
 
         int prfxcount = file.prefixCount();