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;
 	}
 }
 

app/render/worker/workermain.cpp

@@ -24,6 +24,7 @@
 #include <optional>
 
 #include <QFile>
+#include <QFileInfo>
 #include <QGuiApplication>
 #include <QJsonDocument>
 #include <QJsonObject>
@@ -263,6 +264,22 @@ private:
 
 	bool LoadGraph(const QString &path)
 	{
+		{
+			QFileInfo fi(path);
+			if (!fi.exists()) {
+				LogError(QStringLiteral("LoadGraph: graph file does not exist: %1").arg(path));
+				return Write(ErrorMessage(QStringLiteral("graph file does not exist: %1").arg(path)));
+			}
+			if (fi.size() == 0) {
+				LogError(QStringLiteral("LoadGraph: graph file is empty: %1").arg(path));
+				return Write(ErrorMessage(QStringLiteral("graph file is empty: %1").arg(path)));
+			}
+			LogError(QStringLiteral("LoadGraph: loading %1 (%2 bytes, readable=%3)")
+					 .arg(path)
+					 .arg(fi.size())
+					 .arg(fi.isReadable()));
+		}
+
 		auto loaded = std::make_unique<olive::Project>();
 		// Do not call Initialize() here: project serializers expect a blank
 		// project (root_ == nullptr) and will set root themselves. Calling