Try following
="1.0"
<configuration>
<system.web>
.. existing text ..
.. existing text ..
</system.web>
<system.webServer>
<httpErrors>
<remove statusCode="401" subStatusCode="-1" />
<remove statusCode="403" subStatusCode="-1" />
<remove statusCode="404" subStatusCode="-1" />
<remove statusCode="500" subStatusCode="-1" />
<error statusCode="401" path="http://foo.com/default.htm" responseMode="Redirect" />
<error statusCode="403" path="/errors/403.htm" responseMode="ExecuteURL" />
<error statusCode="404" path="/somedir/oops404.htm" responseMode="ExecuteURL" />
<error statusCode="500" path="/somedir/500.asp" responseMode="ExecuteURL" />
</httpErrors>
<modules runAllManagedModulesForAllRequests="true"/>
</system.webServer>
</configuration>
Above code snippet copied from following link
web-config-custom-httperrors[
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