added error messages for the case that no ztree or zleaf argument is in the config

src/Lib/settings.cpp

@@ -45,8 +45,8 @@ lc::Settings::Settings( pt::ptree root ) :
                                            "The display of preprints will not work."))),
   killallCmd(QString::fromStdString(binary_paths.get<std::string>("killall_command",
                                              "Killing 'zleaf.exe' or 'ztree.exe' instances will not work."))),
-  zleafExeName(QString::fromStdString(binary_paths.get<std::string>("zleaf_exe_name"))),
-  ztreeExeName(QString::fromStdString(binary_paths.get<std::string>("ztree_exe_name"))),
+  zleafExeName(QString::fromStdString(binary_paths.get<std::string>("zleaf_exe_name", "using zleaf will not work."))),
+  ztreeExeName(QString::fromStdString(binary_paths.get<std::string>("ztree_exe_name", "using ztree will not work."))),
   latexCmd(QString::fromStdString(binary_paths.get<std::string>("latex_command",
                                            "Receipts creation will not work."))),
   lcDataDir(QString::fromStdString(binary_paths.get<std::string>("labcontrol_data_directory",