[SalesForce] How to catch Validation error in Flow

I have created flow + Process Builder to do a lookup relationship Count on parent object. My issues is I have a validation rule which fires when user try to deleted last child record from the related list. The validation rule is firing as an error. I don't know how to catch this error and display it in the page. There should not be any visualforce page is my requirement.

trigger JobApplicationTrigger on Job_Application__c (after delete) {
    try
    {
        for(Job_Application__c jobapp : Trigger.old)
        {
           Map<String, object> param= new Map<String,object>();
           param.put('getRecordID',jobapp.Id);
           param.put('varCandidateID',jobapp.Candidate__c);
           Flow.Interview.DeleteJobAppRec_flow flow = new Flow.Interview.DeleteJobAppRec_flow(param);
           flow.start();
        }
    }
    catch(DMLException err)
    {
      new Job_Application__c().addError('You got an Error :'+err);   
    }
}