time triggert function, cell error handling - google-apps-script

Question

The time triggered function I'm calling got a fixed range. It validates this range and sends an email if necessary. This works so far but sometimes I got just errors. I assume the content of the range is not loaded, but I'm not sure. With your help, I want to improve the error message I got or figure out how to solve that problem.
sendMailToCheckedMembers is the time triggert function
getMembersFromRange gets the values of the cells, returned as a map
function sendMailToCheckedMembers(){
var ss = SpreadsheetApp.getActiveSpreadsheet();
var sheet = ss.getSheetByName(MAIN_PAGE);
var rangeToCheck = sheet.getRange("L5:P28");
var map = getMembersFromRange(rangeToCheck);
//check for errors
for (var ele in map) {
if(isErrorOrLoading(ele)){
sendError(createStringFromMap(map));
return;
}
}
//do validation stuff
//send validation mail
}
function getMembersFromRange(range){
var numRows = range.getNumRows();
var numCols = range.getNumColumns();
var map = {};
for (var i = 1; i <= numRows; i++) {
for (var j = 1; j <= numCols; j++) {
var currentCell = range.getCell(i,j).getValue();
var entryArray = currentCell.split(" ");
for(var k=0;k<entryArray.length;k++){
map[entryArray[k]] = entryArray[k];
}
}
}
return map;
}
this line var currentCell = range.getCell(i,j).getValue(); contains sometimes the String #ERROR! but it seams like no further information.
I tried to work with sleep(time) to just wait a few seconds after I found errors and then just try it again, but this does not help at all. I don't know what the error is about.
I'm searching for something like range.getCell(i,j).getErrorText() or a way to just wait for the sheet to finish loading the cell's content.
Like I sad above the expected validation email is send 4 out of 7 days and the other 3 days I just get the error mail.
I hope you have some ideas that a can check.
thanks!
Edit:
The content of the cells are filled with the following custom script:
function conCatDates(rangeAsString,range){
var args = Array.prototype.slice.call(arguments, 1);
var rangeArr = rangeAsString.split(";");
var returnValue =[];
for(var i=0;i<rangeArr.length;i++){
var splited = rangeArr[i].split("!");
var sheetname = splited[0];
if(args[i] != ""){
returnValue.push(sheetname);
}
}
if(returnValue.length==0){
return "";
}
return returnValue.sort().join(" ");
}
conCatDates example input conCatDates("Sheetname1!G13;Sheetname2!G13";Sheetname1!G13;Sheetname2!G13), goal of that function is, to get the sheetname of all not empty cells