Use shared method for both POST & GET to output errors

AFAIK, we didn't (still don't) have any GET stuff adding block errors
(AbstractBlock::addError), but we might (should) have some day.

Render requests will so far probably still only fail with flow-error-no-render,
but we might have more specific ->addError() errors in the future.

Change-Id: Ib544b18a8b4fde56cbd92446b95ce8e801ad1ccf
Merge "Use shared method for both POST & GET to output errors"