Fix: [NewGRF] Errors with severity ERROR also display a pop-up window (#9119)

src/lang/english.txt

@@ -3079,6 +3079,7 @@ STR_NEWGRF_ERROR_MSG_WARNING                                    :{RED}Warning: {
 STR_NEWGRF_ERROR_MSG_ERROR                                      :{RED}Error: {SILVER}{RAW_STRING}
 STR_NEWGRF_ERROR_MSG_FATAL                                      :{RED}Fatal: {SILVER}{RAW_STRING}
 STR_NEWGRF_ERROR_FATAL_POPUP                                    :{WHITE}A fatal NewGRF error has occurred: {}{STRING5}
+STR_NEWGRF_ERROR_POPUP                                          :{WHITE}A NewGRF error has occurred: {}{STRING5}
 STR_NEWGRF_ERROR_VERSION_NUMBER                                 :{1:RAW_STRING} will not work with the TTDPatch version reported by OpenTTD
 STR_NEWGRF_ERROR_DOS_OR_WINDOWS                                 :{1:RAW_STRING} is for the {RAW_STRING} version of TTD
 STR_NEWGRF_ERROR_UNSET_SWITCH                                   :{1:RAW_STRING} is designed to be used with {RAW_STRING}

src/newgrf_gui.cpp

@@ -47,8 +47,8 @@ void ShowNewGRFError()
 	if (_game_mode == GM_MENU) return;
 
 	for (const GRFConfig *c = _grfconfig; c != nullptr; c = c->next) {
-		/* We only want to show fatal errors */
-		if (c->error == nullptr || c->error->severity != STR_NEWGRF_ERROR_MSG_FATAL) continue;
+		/* Only show Fatal and Error level messages */
+		if (c->error == nullptr || (c->error->severity != STR_NEWGRF_ERROR_MSG_FATAL && c->error->severity != STR_NEWGRF_ERROR_MSG_ERROR)) continue;
 
 		SetDParam   (0, c->error->message != STR_NULL ? c->error->message : STR_JUST_RAW_STRING);
 		SetDParamStr(1, c->error->custom_message.c_str());
@@ -57,7 +57,11 @@ void ShowNewGRFError()
 		for (uint i = 0; i < lengthof(c->error->param_value); i++) {
 			SetDParam(4 + i, c->error->param_value[i]);
 		}
-		ShowErrorMessage(STR_NEWGRF_ERROR_FATAL_POPUP, INVALID_STRING_ID, WL_CRITICAL);
+		if (c->error->severity == STR_NEWGRF_ERROR_MSG_FATAL) {
+			ShowErrorMessage(STR_NEWGRF_ERROR_FATAL_POPUP, INVALID_STRING_ID, WL_CRITICAL);
+		} else {
+			ShowErrorMessage(STR_NEWGRF_ERROR_POPUP, INVALID_STRING_ID, WL_ERROR);
+		}
 		break;
 	}
 }