MP Lobby: avoid unnecessary cache rebuild if no required addons were …

…installed

I was unconditionally reloading the cache even if there were a reason ad_hoc_addon_fetch_session
failed. The cache will no longer be reloaded if said function didn't actually install anything.

Fixes #2477.

src/gui/dialogs/multiplayer/lobby.cpp

@@ -262,9 +262,7 @@ bool handle_addon_requirements_gui(const std::vector<mp::game_info::required_add
 
 		if(gui2::show_message(e_title, err_msg, message::yes_no_buttons, true) == gui2::window::OK) {
 			// Begin download session
-			ad_hoc_addon_fetch_session(needs_download);
-
-			return true;
+			return ad_hoc_addon_fetch_session(needs_download);
 		}
 	}