(svn r6464) -Fix r6393: killed a warning when compiling without asserts

added an error popup in the game if autoreplace fails to refit
   Since it's only triggered by conditions bugs can trigger, it works kind of like a non-fatal assert in builds without asserts

lang/english.txt

@@ -2906,6 +2906,8 @@ STR_REPLACE_REMOVE_WAGON_HELP                                   :{BLACK}Make aut
 STR_REPLACE_ENGINE_WAGON_SELECT                                 :{BLACK}Replacing: {ORANGE}{SKIP}{SKIP}{STRING}
 STR_REPLACE_ENGINE_WAGON_SELECT_HELP                            :{BLACK} EXPERIMENTAL FEATURE {}Switch between engine and wagon replacement windows.{}Wagon replacement will only be done if the new wagon can be refitted into carrying the same type of cargo as the old one. This is checked for each wagon when the actual replacement takes place.
 STR_ENGINE_NOT_BUILDABLE                                        :{WHITE}Engine is not buildable
+STR_AUTOREPLACE_FAILED                                          :{WHITE}BUG: Autoreplace refit failure
+STR_AUTOREPLACE_REFIT_FAILURE                                   :{WHITE}{STRING} replaced into {STRING} failed to refit to {STRING}
 
 STR_ENGINES                                                     :Engines
 STR_WAGONS                                                      :Wagons

vehicle.c

@@ -1778,9 +1778,18 @@ static int32 ReplaceVehicle(Vehicle **w, byte flags, int32 total_cost)
 
 		/* refit if needed */
 		if (replacement_cargo_type != CT_NO_REFIT) {
-			int32 temp_cost = DoCommand(0, new_v->index, replacement_cargo_type, DC_EXEC, CMD_REFIT_VEH(new_v->type));
-			assert(!CmdFailed(temp_cost)); // assert failure here indicates a bug in GetNewCargoTypeForReplace()
+			if (CmdFailed(DoCommand(0, new_v->index, replacement_cargo_type, DC_EXEC, CMD_REFIT_VEH(new_v->type)))) {
+				/* We should not be here
+				 * being here shows a failure indicates a bug in GetNewCargoTypeForReplace() or incorrect estimation costs */
+				SetDParam(0, GetCustomEngineName(old_v->engine_type));
+				SetDParam(1, GetCustomEngineName(new_v->engine_type));
+				SetDParam(2, _cargoc.names_s[replacement_cargo_type]);
+				ShowInfo("Error: Autoreplace failed to refit (bug)");
+				ShowErrorMessage(STR_AUTOREPLACE_REFIT_FAILURE, STR_AUTOREPLACE_FAILED, 0 ,0);
+				NOT_REACHED(); // stop debug builds at this error for easier debugging
+			}
 		}
+
 		if (new_v->type == VEH_Train && HASBIT(old_v->u.rail.flags, VRF_REVERSE_DIRECTION) && !IsMultiheaded(new_v) && !(new_v->next != NULL && IsArticulatedPart(new_v->next))) {
 			// we are autorenewing to a single engine, so we will turn it as the old one was turned as well
 			SETBIT(new_v->u.rail.flags, VRF_REVERSE_DIRECTION);