render: add diagnostics for worker graph load failures

- Report QFile::errorString() when ProjectSerializer::Load() fails to open
  the graph file, so the worker log shows why instead of an empty detail.
- In olive-render-worker LoadGraph(), check file existence, size and
  readability before delegating to the serializer and log the attempt.

app/node/project/serializer/serializer.cpp

@@ -96,7 +96,10 @@ ProjectSerializer::Result ProjectSerializer::Load(Project *project,
 			return inner_result;
 		}
 	} else {
-		return kFileError;
+		Result r(kFileError);
+		r.SetDetails(QStringLiteral("Unable to open '%1': %2")
+					 .arg(filename, project_file.errorString()));
+		return r;
 	}
 }