net/nanocoap: add debug message for server

[kb2ma]

Contribution description

Adds a debug message to the nanocoap_server() function when the result of generating the response indicates an error has occurred.

Testing procedure

You can add a failing handler function to the nanocoap_server example's coap_handler.c. For example:

static ssize_t _fail(coap_pkt_t *pkt, uint8_t *buf, size_t len, void *context)
{
    (void)pkt;
    (void)buf;
    (void)len;
    (void)context;
    return -1;
}

Also add _fail to the coap_resources array at the bottom of the file, for example as the resource /fail. Of course, you'll also want to enable debugging in sock.c. :-)

Finally, start a couple of native instances -- nanocoap_server and gcoap. Then have the gcoap instance request the /fail resource from the nanocoap_server instance. In the nanocoap terminal you should see:

error handling request -1

Issues/PRs references

-none-

[kaspar030]

ACK.

[kaspar030]

The extra message makes sense.

At some point we should maybe think about using LOG_*...

[kb2ma]

Thanks for the quick approval. This PR will be helpful for testing the next one. ;-)