jfjoch_viewer: Make code tighter regarding empty images

reader/JFJochHttpReader.cpp

@@ -10,15 +10,10 @@
 #include "../broker/gen/model/Plots.h"
 #include "../image_analysis/bragg_integration/CalcISigma.h"
 
-void JFJochHttpReader::Close_i() {
-    addr = "";
-    SetStartMessage({});
-
-}
-
 void JFJochHttpReader::Close() {
     std::unique_lock ul(http_mutex);
-    Close_i();
+    addr = "";
+    SetStartMessage({});
 }
 
 uint64_t JFJochHttpReader::GetNumberOfImages() const {
@@ -43,14 +38,21 @@ uint64_t JFJochHttpReader::GetNumberOfImages() const {
     }
 }
 
-std::shared_ptr<JFJochReaderDataset>  JFJochHttpReader::UpdateDataset_i() {
+std::shared_ptr<JFJochReaderDataset> JFJochHttpReader::UpdateDataset_i() {
     httplib::Client cli_cmd(addr);
 
     auto res = cli_cmd.Get("/image_buffer/start.cbor");
 
-    if (!res || res->status != httplib::StatusCode::OK_200)
+    if (!res || (res->status != httplib::StatusCode::OK_200
+        && res->status != httplib::StatusCode::NotFound_404))
         throw JFJochException(JFJochExceptionCategory::InputParameterInvalid,
                               "Could not get image buffer status");
+
+    if (res->status == httplib::StatusCode::NotFound_404)
+        return {};
+    if (res->body.empty())
+        return {};
+
     try {
         auto msg = CBORStream2Deserialize(res->body);
 
@@ -125,19 +127,18 @@ std::shared_ptr<JFJochReaderDataset>  JFJochHttpReader::UpdateDataset_i() {
 
         return dataset;
     } catch (std::exception &e) {
-        Close_i();
         throw JFJochException(JFJochExceptionCategory::InputParameterInvalid,
-                              "Could not load dataset");
+                              std::string("Could not load dataset: ") + std::string(e.what()));
     }
 }
 
 void JFJochHttpReader::ReadURL(const std::string &url) {
     std::unique_lock ul(http_mutex);
-
     addr = url;
     if (url.empty())
-        Close_i();
-    SetStartMessage(UpdateDataset_i());
+        SetStartMessage({});
+    else
+        SetStartMessage(UpdateDataset_i());
 }
 
 bool JFJochHttpReader::LoadImage_i(std::shared_ptr<JFJochReaderDataset> &dataset,
@@ -155,6 +156,8 @@ bool JFJochHttpReader::LoadImage_i(std::shared_ptr<JFJochReaderDataset> &dataset
     // Always update dataset, as it might have changed from the last time
     if (update_dataset)
         dataset = UpdateDataset_i();
+    if (!dataset)
+        return false;
 
     auto res = cli_cmd.Get("/image_buffer/image.cbor?id=" + std::to_string(image_number));