How to redirect the user to the friendly error-handler page when an Application error occurs

Modify web.config as

<customErrors mode='On' defaultRedirect='errorpage.aspx' />

The <customErrors> configuration section supports an inner <error> tag that associates HTTP status codes with custom error pages. For example:

<customErrors mode='On' defaultRedirect='genericerror.htm'>
	<error statusCode='404' redirect='pagenotfound.aspx'/>
	<error statusCode='403' redirect='noaccess.aspx'/>
</customErrors>

For more details refer <customErrors> Element