fix return code for render_handler

[Thorsieger]

Hello,

carbonapi do not response correct code 404 when no metric is found from gocarbon/carbonserver when requesting for a render

This should fix the issue :

returnCode is reasigned in an other scope, so any modification won't have effect ouside the if

[msaf1980]

Hello,

carbonapi do not response correct code 404 when no metric is found from gocarbon/carbonserver when requesting for a render

This should fix the issue :

returnCode is reasigned in an other scope, so any modification won't have effect ouside the if

@Civil May be extend e2e tests for check status code ? Some of them are cheched, but this case not.