load_plugins: Preserve original exception with failure information

The original implementation threw an exception with a list of plugins
that failed to load. Usually we got the name of one configuration file.
Information about the reason for the failure was not included
in the exception.

Now the code throws an exception on the first failed plugin. This
exception includes the original exception from the source of the error.

dnf5/plugins.cpp

@@ -161,21 +161,14 @@ void Plugins::load_plugins(const std::string & dir_path) {
     }
     std::sort(lib_paths.begin(), lib_paths.end());
 
-    std::string failed_filenames;
     for (const auto & path : lib_paths) {
         try {
             load_plugin(path);
         } catch (const std::exception & ex) {
             logger->error("Cannot load dnf5 plugin \"{}\": {}", path.string(), ex.what());
-            if (!failed_filenames.empty()) {
-                failed_filenames += ", ";
-            }
-            failed_filenames += path.filename();
+            std::throw_with_nested(std::runtime_error("Cannot load dnf5 plugin: " + path.filename().string()));
         }
     }
-    if (!failed_filenames.empty()) {
-        throw std::runtime_error("Cannot load dnf5 plugins: " + failed_filenames);
-    }
 }
 
 bool Plugins::init() {

libdnf5/plugin/plugins.cpp

@@ -245,23 +245,15 @@ void Plugins::load_plugins(
     }
     std::sort(config_paths.begin(), config_paths.end());
 
-    std::string failed_filenames;
     for (const auto & path : config_paths) {
         try {
             load_plugin(path, plugin_enablement);
         } catch (const std::exception & ex) {
             logger.error("Cannot load plugin \"{}\": {}", path.string(), ex.what());
-            if (!failed_filenames.empty()) {
-                failed_filenames += ", ";
-            }
-            failed_filenames += path.filename();
+            std::throw_with_nested(PluginError(M_("Cannot load plugin: {}"), path.filename().string()));
         }
     }
 
-    if (!failed_filenames.empty()) {
-        throw PluginError(M_("Cannot load plugins: {}"), failed_filenames);
-    }
-
     // Creates a PluginInfo for each loaded plugin.
     auto & plugins_info = InternalBaseUser::get_plugins_info(base);
     for (const auto & plugin : plugins) {