LibCore: Make Core::File::open() return a Result<NNRP<File>, String>

It was impractical to return a RefPtr<File> since that left us no way
to extract the error string. This is usually needed for the UI, so the
old static open() got basically no use.

Libraries/LibCore/File.cpp

@@ -33,11 +33,11 @@
 
 namespace Core {
 
-RefPtr<File> File::open(const String& filename, IODevice::OpenMode mode, mode_t permissions)
+Result<NonnullRefPtr<File>, String> File::open(const String& filename, IODevice::OpenMode mode, mode_t permissions)
 {
     auto file = File::construct(filename);
     if (!file->open_impl(mode, permissions))
-        return nullptr;
+        return String(file->error_string());
     return file;
 }