Add error messages for vehicle build failure due to wrong depot owner and rail type

src/lang/english.txt

@@ -5429,6 +5429,9 @@ STR_ERROR_NO_VEHICLES_AVAILABLE_AT_ALL_EXPLANATION              :{WHITE}Change y
 STR_ERROR_NO_VEHICLES_AVAILABLE_YET                             :{WHITE}No vehicles are available yet
 STR_ERROR_NO_VEHICLES_AVAILABLE_YET_EXPLANATION                 :{WHITE}Start a new game after {DATE_SHORT} or use a NewGRF that provides early vehicles
 
+STR_ERROR_CANT_PURCHASE_OTHER_COMPANY_DEPOT                     :{WHITE}Depot is owned by another company, and building vehicles there is not allowed.
+STR_ERROR_DEPOT_HAS_WRONG_RAIL_TYPE                             :{WHITE}Depot cannot be used to build trains with this rail type.
+
 # Specific vehicle errors
 STR_ERROR_CAN_T_MAKE_TRAIN_PASS_SIGNAL                          :{WHITE}Can't make train pass signal at danger...
 STR_ERROR_CAN_T_REVERSE_DIRECTION_TRAIN                         :{WHITE}Can't reverse direction of train...

src/train_cmd.cpp

@@ -940,7 +940,7 @@ CommandCost CmdBuildRailVehicle(TileIndex tile, DoCommandFlag flags, const Engin
 
 	/* Check if depot and new engine uses the same kind of tracks *
 	 * We need to see if the engine got power on the tile to avoid electric engines in non-electric depots */
-	if (!HasPowerOnRail(rvi->railtype, GetRailType(tile))) return CMD_ERROR;
+	if (!HasPowerOnRail(rvi->railtype, GetRailType(tile))) return_cmd_error(STR_ERROR_DEPOT_HAS_WRONG_RAIL_TYPE);
 
 	if (flags & DC_EXEC) {
 		DiagDirection dir = GetRailDepotDirection(tile);

src/vehicle_cmd.cpp

@@ -95,10 +95,10 @@ CommandCost CmdBuildVehicle(TileIndex tile, DoCommandFlag flags, uint32 p1, uint
 
 	VehicleType type = GetDepotVehicleType(tile);
 	if (!IsTileOwner(tile, _current_company)) {
-		if (!_settings_game.economy.infrastructure_sharing[type]) return CMD_ERROR;
+		if (!_settings_game.economy.infrastructure_sharing[type]) return_cmd_error(STR_ERROR_CANT_PURCHASE_OTHER_COMPANY_DEPOT);
 
 		const Company *c = Company::GetIfValid(GetTileOwner(tile));
-		if (c == nullptr || !c->settings.infra_others_buy_in_depot[type]) return CMD_ERROR;
+		if (c == nullptr || !c->settings.infra_others_buy_in_depot[type]) return_cmd_error(STR_ERROR_CANT_PURCHASE_OTHER_COMPANY_DEPOT);
 	}
 
 	/* Validate the engine type. */