Custom Error Message for Event Receiver in SharePoint 2010 -

i want users upload .doc files in document library.

to so, have developed event receiver in visual studio 2010.

my code follows:

public override void itemadding(spitemeventproperties properties) {        try        {             base.itemadding(properties);            eventfiringenabled = false;             if (!properties.afterurl.endswith("doc"))            {                properties.errormessage = "you allowed updload .doc files";                properties.status = speventreceiverstatus.cancelwitherror;                properties.cancel = true;             }        }        catch (exception ex)        {            properties.status = speventreceiverstatus.cancelwitherror;            properties.errormessage = ex.message.tostring();            properties.cancel = true;        }  } 

the code referred this example.

my problem while uploading non-doc files preventing system error message not user friendly defined in properties.errormessage.

how solve this?

please help.

i used same code have provided in question, custom error message displayed shown in below image -

please provide details of error getting.