mp: Improve display of wesnothd client errors in general

Besides replacing the "End of file" error when getting disconnected from
the server under unexpected circumstances (e.g. because the server died)
with a translatable and more intuitive message, this also makes it so
other network error messages ("Connection refused", "Host not found",
etcetera) are displayed in the UI in a slightly clearer fashion, and in
an error dialog that must be dismissed with a click on a button, instead
of a transient message that can be easily missed due to an accidental
misclick.

Closes #3005.

(cherry-picked from commit c503c2ce35)

changelog.md

@@ -65,6 +65,7 @@
    * Removed individual Join/Observe buttons for each game in the MP Lobby.
    * Highlight the titles of MP games with vacant slots.
    * Improved MP Lobby layout on low resolutions.
+   * Improved reporting of network errors in the MP lobby (issue #3005).
  ### WML engine
    * Support formula= key in [variable] ConditionalWML
    * Support to_location in [move_unit], taking a location ID

players_changelog.md

@@ -12,6 +12,7 @@ changelog: https://github.com/wesnoth/wesnoth/blob/master/changelog.md
      Spanish, Ukrainian.
  ### User Interface
    * Implemented MP chat message history saving (issue #1194, issue #2802).
+   * Improved reporting of network errors in the MP lobby (issue #3005).
  ### Miscellaneous and bug fixes
    * Fixed minimap buttons not doing anything (bug #2681)
    * Fixed some hotkeys triggering multiple commands on GNU/Linux (bug #1736)

src/game_launcher.cpp

@@ -867,7 +867,26 @@ bool game_launcher::play_multiplayer(mp_selection res)
 	} catch(wesnothd_error& e) {
 		if(!e.message.empty()) {
 			ERR_NET << "caught network error: " << e.message << std::endl;
-			gui2::show_transient_message("", translation::gettext(e.message.c_str()));
+
+			std::string user_msg;
+			auto conn_err = dynamic_cast<wesnothd_connection_error*>(&e);
+
+			if(conn_err) {
+				// The wesnothd_connection_error subclass is only thrown with messages
+				// from boost::system::error_code which we can't translate ourselves.
+				// It's also the originator of the infamous EOF error that happens when
+				// the server dies. <https://github.com/wesnoth/wesnoth/issues/3005>. It
+				// will provide a translated string instead of that when it happens.
+				user_msg = !conn_err->user_message.empty()
+				           ? conn_err->user_message
+				           : _("Connection failed: ") + e.message;
+			} else {
+				// This will be a message from the server itself, which we can
+				// probably translate.
+				user_msg = translation::gettext(e.message.c_str());
+			}
+
+			gui2::show_error_message(user_msg);
 		} else {
 			ERR_NET << "caught network error" << std::endl;
 		}

src/wesnothd_connection.cpp

@@ -15,6 +15,7 @@
 #include "wesnothd_connection.hpp"
 
 #include "gui/dialogs/loading_screen.hpp"
+#include "gettext.hpp"
 #include "log.hpp"
 #include "serialization/parser.hpp"
 #include "utils/functional.hpp"
@@ -396,7 +397,13 @@ bool wesnothd_connection::receive_data(config& result)
 	{
 		std::lock_guard<std::mutex> lock(last_error_mutex_);
 		if(last_error_) {
-			throw error(last_error_);
+			std::string user_msg;
+
+			if(last_error_ == boost::asio::error::eof) {
+				user_msg = _("Disconnected from server.");
+			}
+
+			throw error(last_error_, user_msg);
 		}
 	}
 

src/wesnothd_connection_error.hpp

@@ -46,6 +46,13 @@ struct ingame_wesnothd_error : public wesnothd_error ,public lua_jailbreak_excep
 ///TODO: find a short name
 struct wesnothd_connection_error : public wesnothd_error ,public lua_jailbreak_exception
 {
-	wesnothd_connection_error(const boost::system::error_code& error) : wesnothd_error(error.message()) {}
+	wesnothd_connection_error(const boost::system::error_code& error, const std::string& msg = "")
+		: wesnothd_error(error.message())
+		, user_message(msg)
+	{}
+
+	/** User-friendly and potentially translated message for use in the UI. */
+	std::string user_message;
+
 	IMPLEMENT_LUA_JAILBREAK_EXCEPTION(wesnothd_connection_error)
 };