Chunnel: Add error text for too many tunnels.

src/lang/english.txt

@@ -5006,6 +5006,7 @@ STR_ERROR_TUNNEL_THROUGH_MAP_BORDER                             :{WHITE}Tunnel w
 STR_ERROR_UNABLE_TO_EXCAVATE_LAND                               :{WHITE}Unable to excavate land for other end of tunnel
 STR_ERROR_TUNNEL_TOO_LONG                                       :{WHITE}... tunnel too long
 STR_ERROR_TUNNEL_RAMP_TOO_SHORT                                 :{WHITE}... ramp too short, tunnels under water must have a ramp at least three tiles long at both ends.
+STR_ERROR_TUNNEL_TOO_MANY                                       :{WHITE}... too many tunnels
 STR_ERROR_NO_DRILLING_ABOVE_CHUNNEL                             :{WHITE}No oil rigs allowed above underwater tunnels.
 STR_ERROR_ANOTHER_TUNNEL_IN_THE_WAY_FOR_CHUNNEL                 :{WHITE}Three tiles are needed to pass under the other tunnel.
 

src/saveload/afterload.cpp

@@ -2038,7 +2038,12 @@ bool AfterLoadGame()
 					TileIndex start_tile = t;
 					TileIndex end_tile = GetOtherTunnelBridgeEndOld(start_tile);
 
-					if (!Tunnel::CanAllocateItem()) return false;
+					if (!Tunnel::CanAllocateItem()) {
+						SetSaveLoadError(STR_ERROR_TUNNEL_TOO_MANY);
+						/* Restore the signals */
+						ResetSignalHandlers();
+						return false;
+					}
 
 					const Tunnel *t = new Tunnel(start_tile, end_tile, false);
 

src/tunnelbridge_cmd.cpp

@@ -813,7 +813,7 @@ CommandCost CmdBuildTunnel(TileIndex start_tile, DoCommandFlag flags, uint32 p1,
 		TileIndex ts = end_tile;
 		if(start_tile > end_tile) Swap(tn, ts);
 
-		if (!Tunnel::CanAllocateItem()) return CMD_ERROR;
+		if (!Tunnel::CanAllocateItem()) return_cmd_error(STR_ERROR_TUNNEL_TOO_MANY);
 		const Tunnel *t = new Tunnel(tn, ts, is_chunnel);
 
 		if (transport_type == TRANSPORT_RAIL) {