The task is convert a classic button into lightning action. The existing apex controller is unchanged except for adding @AuraEnabled for webservice that will return a list to the client side controller. There are 3 scenarios - the client side controller will alert based on the message received from the apex controller:

There are 2 failure scenarios –  able to display them as alerts
There is 1 success scenario – The alert is not triggering for this case.

Apex controller:

global without sharing class giftBatchHelper {

    public boolean tagPendingGifts(Id BatchId){
        boolean ret = true;
        this.errorDesc = '';
        this.numOpps = 0;
        try{
            if (string.isNotBlank(BatchId)){
               List<Opportunity> lstOppCheck = [select Id from Opportunity where Gift_Batch__c = :BatchId];
               if (lstOppCheck != null)
                  if (lstOppCheck.size() > 0) {
                     this.errorDesc = 'This batch already has Gifts assigned.  You may not run the batch assignment process more than once for a given Gift Batch.';
                     ret = false;
                     return ret;
                  }
               this.errorDesc = 'There are no Posted Opportunities without a Gift Batch assignment.';
               List<Opportunity> lstOppUPdate = [select Id, Gift_Batch__c  from Opportunity where Gift_Batch__c = null AND StageName = 'Posted' AND RecordType.Name = 'Donation'];
               if (lstOppUPdate != null)
                  if (lstOppUPdate.size() > 0) {
                     for (Opportunity o: lstOppUPdate ) 
                        o.Gift_Batch__c = BatchId;
                     Update lstOppUPdate;
                     this.numOpps = lstOppUPdate.size();
                     this.errorDesc = '';
                  }
            }  
        }
        catch(Exception exc){
            this.errorDesc = exc.getMessage();
            ret = false;
        }
        return ret;
    }
@AuraEnabled
    webservice static list<string> TagPendingGiftsForBatch(string strBatchId){    
        Id BatchId = Id.valueOf(strBatchId);
        if (BatchId ==null) return new list<string>{'Error: Invalid Batch Id',''};
        giftBatchHelper objgiftBatchHelper  = new giftBatchHelper();
        objgiftBatchHelper.tagPendingGifts(BatchId);
        return new list<string>{objgiftBatchHelper.errorDesc,string.valueOf(objgiftBatchHelper.numOpps)};
    }

}

Client side controller:

({
    "echo" : function(cmp) {
        // create a one-time use instance of the serverEcho action
        // in the server-side controller
        var action = cmp.get("c.TagPendingGiftsForBatch");

        var varRecordId = cmp.get("v.recordId");
        //alert('varRecordId: ' + varRecordId);
        action.setParams({  strBatchId : varRecordId  });
        
        //alert('before callback');
        
        action.setCallback(this, function(response) {
            
            alert('inside call back');
            var result = response.getReturnValue();
            //if(response.getReturnValue() != null)
            alert('result[0] ' + result[0]);
            alert('result[1] ' + result[1]);
            if(result[1]==0)
            {
                var result = response.getReturnValue();
                //var retMessage = result[0];
                //alert('result[1] ' + result[1]);
                //alert('result[0].length ' + result[0].length);
                //alert('result.length' + result.length);
            
                if(result.length === 2)
                {
                    if(result[0].length > 0) 
                    {
                        alert(result[0]);                         
                    }
                }
            }
            else if(result[1]>0)
            {
                alert('Gift(s) updated with batch:');
            }
            else
            {
                alert('Error: Batch Id is empty.');
            }
            });
        
        $A.enqueueAction(action);
        $A.get("e.force:closeQuickAction").fire();
    }
})


The bold italics section should work when there is a value in result[1] - however, the flow is getting inside the setCallback i.e. the alert('inside call back'); doesn't throw the alert.

Please let me know where there issue is...