<httpErrors errorMode="Custom" existingResponse="Replace">
      <remove statusCode="404" subStatusCode="-1" />
      <error statusCode="404" path="/404.aspx" responseMode="ExecuteURL"/>
    </httpErrors>

<httpErrors errorMode="Custom" existingResponse="Auto" defaultResponseMode="ExecuteURL" defaultPath="/App/Error"> <!-- Do not include ~, this was my issue all long -->
  <clear/> <!-- so that IIS provided error pages are skipped -->
  <!-- add those which you like to provide a view of yours -->
  <error path="/App/Http404" responseMode="ExecuteURL" statusCode="404"/>
  <error path="/App/Http503" responseMode="ExecuteURL" statusCode="503"/>
</httpErrors>

<customErrors mode="On" defaultRedirect="~/DefaultError.aspx?msg=SomeMessage">
  <error statusCode="404" redirect="~/PageNotFound.html"/>
  <error statusCode="403" redirect="~/AccessDenied.html"/>
</customErrors>