How to show custom message on page instead of error message while . reloading the page?

<apex:page standardController="Account" extensions="myaccountcontroller">
<apex:form >
<apex:pagemessages ></apex:pagemessages>
    <apex:commandButton value="Save" action="{!Save}"/>
</apex:form>
</apex:page>
public class myaccountcontroller
{
  public string accId {get;set;}
  public String finalamount {get;set;}
  public myaccountcontroller(ApexPages.StandardController sc)
  {
    accId = ApexPages.CurrentPage().getParameters().get('Id');
    doCalculation();
  }
  public void doCalculation()
  {
    Account AccObj = [SELECT Id,Name,Amount__c,Total_Amount__c FROM Account WHERE Id=:accId ];
    system.debug('TotalAmount ⌗⌗⌗:'+AccObj.Total_Amount__c );
   try{
        finalamount  = String.ValueOf(AccObj.Amount__c/0);
        system.debug('finalamount⌗⌗⌗:'+finalamount);
     }catch(Exception e)
     {
        if(e.getMessage() == 'System.NullPointerException: Attempt to de-reference a null object')
        {
          ApexPages.addmessage(new ApexPages.message(ApexPages.severity.ERROR,'Unite Price should not be empty'));
        }
     }
  }
}