We can configure html pages for each unhandled error which occured during the execution of web Request in <customErrors> section of web.config file .
<configuration>
<system.web>
<customErrors mode="RemoteOnly" defaultRedirect="errorpage.aspx">
<error statusCode="403" redirect="errorpage403.aspx"/>
<error statusCode="404" redirect="errorpage404.aspx"/>
</customErrors>
</system.web>
</configuration>
