Send to connect: trying to extract server error message for nicer err…

…or dialog

src/slic3r/Utils/PrusaConnect.cpp

@@ -46,6 +46,24 @@ std::string escape_path_by_element(const boost::filesystem::path& path)
     }
     return ret_val;
 }
+
+boost::optional<std::string> get_error_message_from_response_body(const std::string& body)
+{
+    boost::optional<std::string> message;
+    std::stringstream ss(body);
+    pt::ptree ptree;
+    try
+    {
+        pt::read_json(ss, ptree);
+        message = ptree.get_optional<std::string>("message");
+    }
+    // ignore possible errors if body is not valid JSON
+    catch (std::exception&)
+    {}
+
+    return message;
+}
+
 }
 
 PrusaConnectNew::PrusaConnectNew(DynamicPrintConfig *config) 
@@ -125,7 +143,9 @@ bool PrusaConnectNew::init_upload(PrintHostUpload upload_data, std::string& out)
             BOOST_LOG_TRIVIAL(error) << body;
             BOOST_LOG_TRIVIAL(error) << boost::format("%1%: Error registering file: %2%, HTTP %3%, body: `%4%`") % name % error % status % body;
             res = false;
-            out = GUI::into_u8(format_error(body, error, status));
+            out = get_error_message_from_response_body(body).value_or_eval([&](){
+                return GUI::into_u8(format_error(body, error, status));
+            });
         })
         .perform_sync();
     return res;