Suggest checking the data dir path when unable to initialize fonts

Not being able to initialize fonts is usually the direct consequence of
a failure to read data/hardwired/fonts.cfg, possibly because the game
data dir was incorrectly set or detected. Thus, it's the ideal point
during early initialization to provide a hint on stderr for players.

I specifically avoided mentioning anything about "installing" the game
so as to not contribute to the common misconception that the game needs
to be installed to run.

changelog

@@ -53,6 +53,8 @@ Version 1.11.15+dev:
    * Fix bug #22147: Assertion error when a network error is thrown during the game.
    * Removed the YetAnotherMapGenerator (not finished)
    * Fix bug #22134: Campaign prefix not used in mp campaign saves
+   * Made the error messages sent to stderr when the core data dir is
+     incorrectly set more helpful.
 
 Version 1.11.15:
  * Graphics:

src/game.cpp

@@ -414,6 +414,15 @@ static void init_locale() {
 	textdomain (PACKAGE);
 }
 
+static void warn_early_init_failure()
+{
+	// NOTE: wrap output to 80 columns.
+	std::cerr << '\n'
+			  << "An error at this point during initialization usually indicates that the data\n"
+			  << "directory above was not correctly set or detected. Try passing the correct path\n"
+			  << "in the command line with the --config-dir switch or as the only argument.\n";
+}
+
 /**
  * Setups the game environment and enters
  * the titlescreen or game loops.
@@ -443,6 +452,7 @@ static int do_gameloop(int argc, char** argv)
 	res = font::load_font_config();
 	if(res == false) {
 		std::cerr << "could not initialize fonts\n";
+		warn_early_init_failure();
 		return 1;
 	}