try{
    this.acctURL   = (Account)controller.getRecord();
}catch(system.DMLException e){
    displayIDError = true;
    errorMessage   = 'Salesforce cant find an account with an ID of: ' + acctIDPage + ' Please contact your Salesforce Administrator';
}

this.acctURL = (Account)controller.getRecord();

public class AccountIdChecker
{
    public PageReference Init()
    {
        String acctId = ApexPages.currentPage().getParameters().get('Id');

        if(acctId == '')
        {
            ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.Error, 'No id specified!'));
            return null;
        }

        try
        {
            Id realAcctId = acctId;
            if(Schema.Account.SObjectType != realAcctId.getSobjectType())
            {
                ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.Error, 'Id specified is not an account id!'));
                return null;
            }
        }
        catch (Exception e)
        {
            ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.Error, 'Bad id specified!'));
            return null;
        }

        <<return reference to existing page here>>
    }
}

<apex:page controller="AccountIdChecker" action="{!Init}">
    <apex:pageMessages/>
</apex:page>