net: zoap: Prepare and send error response separately

If error occurs while preparing response to .well-known/core request,
do not send partly filled payload by simply changing code part only.
It's better to unref the partly filled reponse and prepare a new error
response and send.

Change-Id: I28013a4e331cfc6f01de873e80af48f765e49494
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>