Hellfire: Better error if some MPQs are missing

Makes the issue more obvious.

Source/init.cpp

@@ -192,6 +192,13 @@ void init_archives()
 	hfvoice_mpq = init_test_access(paths, "hfvoice.mpq", "DiabloInstall", 8500, FS_PC);
 	hfopt1_mpq = init_test_access(paths, "hfopt1.mpq", "DiabloInstall", 8600, FS_PC);
 	hfopt2_mpq = init_test_access(paths, "hfopt2.mpq", "DiabloInstall", 8610, FS_PC);
+
+	if (gbIsHellfire && (hfmonk_mpq == NULL || hfmusic_mpq == NULL || hfvoice_mpq == NULL))
+	{
+		UiErrorOkDialog("Some Hellfire MPQs are missing", "Not all Hellfire MPQs were found.\nPlease copy all the hf*.mpq files.");
+		app_fatal(NULL);
+	}
+
 	devilutionx_mpq = init_test_access(paths, "devilutionx.mpq", "DiabloInstall", 9000, FS_PC);
 }