How to display error message from progressErrorCollection.getErrors().toString() in Jira screen, scriptrunner postfunction - jira

Question

I have an error message that is thrown when I try to execute a postfunction, the error message is: 
org.ofbiz.core.entity.GenericTransactionException: Commit failed, rollback previously requested by nested transaction.
You can see the error message in this screenshot:
In my code, I am using the line : 
        log.warn("MOUNA here 2 "+progressErrorCollection.getErrors().toString()) which is displaying a more meaninful message to the user which is: fixVersions:Fix Version/s is required. I would like to print this message instead to the user instead of org.ofbiz.core.entity.GenericTransactionException: Commit failed, rollback previously requested by nested transaction.
How can I enforce this? I have tried using 
      UserMessageUtil.error(progressErrorCollection.getErrors().toString()) but it is not working. Anyone knows? Here is my code below: 
package CombineTransitions
import com.onresolve.scriptrunner.runner.util.UserMessageUtil
import com.atlassian.jira.issue.Issue;
import org.apache.log4j.Logger
import com.atlassian.jira.bc.issue.IssueService
import com.atlassian.jira.issue.Issue;
import CombineTransitions.Configuration_CombineTransitions
import com.atlassian.jira.bc.issue.IssueService.IssueResult
import com.atlassian.jira.bc.issue.IssueService.TransitionValidationResult
import com.atlassian.jira.event.type.EventDispatchOption
import  java.util.concurrent.TimeUnit
import  java.util.concurrent.ScheduledExecutorService
import java.util.concurrent.Executors
import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.MutableIssue
import com.atlassian.jira.workflow.WorkflowTransitionUtil
import com.opensymphony.workflow.WorkflowContext;
import com.atlassian.jira.workflow.WorkflowTransitionUtilImpl;
import com.atlassian.jira.util.JiraUtils;
import com.atlassian.jira.issue.MutableIssue;
import com.atlassian.jira.util.ErrorCollection
import com.atlassian.jira.workflow.WorkflowException
def log1 = Logger.getLogger("atlassian-jira.log")
def combineTransitionsHashMap= Configuration_CombineTransitions.getCombineTransitionsHashMap()
def issueStatusTypeId = combineTransitionsHashMap["issueStatusTypeId"] as String;
def transitionFromOpentoFixed = combineTransitionsHashMap["transitionFromOpentoFixed"] as Integer;
def transitionFromFixedToTested = combineTransitionsHashMap["transitionFromFixedToTested"] as Integer;
def transitionFromTestedToCompleted = combineTransitionsHashMap["transitionFromTestedToCompleted"] as Integer;
def fixedStatusName = combineTransitionsHashMap["fixedStatusName"] as String;
def testedStatusName = combineTransitionsHashMap["testedStatusName"] as String;
def completedStatusName = combineTransitionsHashMap["completedStatusName"] as String;
def fixedStatusId = combineTransitionsHashMap["fixedStatusId"] as String;
def testedStatusId = combineTransitionsHashMap["testedStatusId"] as String;
def completedStatusId = combineTransitionsHashMap["completedStatusId"] as String;
if (issue.getStatus().getSimpleStatus().getId().equals(issueStatusTypeId)) {
        def ok = false
         ok=performTransition(transitionFromOpentoFixed, fixedStatusName, fixedStatusId);
         log.warn("MOUNA first boolean "+ok +" "+fixedStatusName)
   if(ok){
         ok= performTransition(transitionFromFixedToTested, testedStatusName, testedStatusId);
         log.warn("MOUNA second boolean "+ok +" "+testedStatusName)
         if(ok){
          ok= performTransition(transitionFromTestedToCompleted, completedStatusName, completedStatusId);
          log.warn("MOUNA third boolean "+ok +" "+completedStatusName)
         }
 
  }
   
   
}
def  performTransition(int transitionToBeDone, String destinationStatus, String statusId ) {
  log.warn("MOUNA  ORIGINAL STATUS "+ issue.getStatus()  + "DEST STATUS "+ destinationStatus)
issue = ComponentAccessor.getIssueManager().getIssueObject(issue.id)
  def currentUser = ComponentAccessor.getJiraAuthenticationContext().getLoggedInUser()
 
  String issueKey = issue.getKey()
  IssueService issueService = ComponentAccessor.getIssueService()
  def issueInputParameters = issueService.newIssueInputParameters()
  issueInputParameters.setComment("Transitioning issue from status "+ issue.getStatus().getName()+" to status "+ destinationStatus );
WorkflowTransitionUtil workflowTransitionUtil = (WorkflowTransitionUtil) JiraUtils.loadComponent(WorkflowTransitionUtilImpl.class)
workflowTransitionUtil.setIssue(issue)
workflowTransitionUtil.setAction(transitionToBeDone)
ErrorCollection errorCollection=workflowTransitionUtil.validate()
if (errorCollection.hasAnyErrors()) {
    log.warn("MOUNA here 1 "+errorCollection.getErrorMessages().toString())
return false;
}else{
 
    ErrorCollection progressErrorCollection=workflowTransitionUtil.progress()
    if(progressErrorCollection.hasAnyErrors()){
        log.warn("MOUNA here 2 "+progressErrorCollection.getErrors().toString())
            //  throw new WorkflowException(progressErrorCollection.getErrors().toString());
      UserMessageUtil.error(progressErrorCollection.getErrors().toString())
   
          return false;
    }else{
      return true;
    }
}
 
 
}