net: lib: coap: Return an error on removing a non-existing observer

If we're parsing a CoAP request with an observe option of '1', but there is
no matching observer, return an error instead of returning a zero.

Signed-off-by: Pieter De Gendt <pieter.degendt@basalte.be>
(cherry picked from commit d5931bae93)

subsys/net/lib/coap/coap_server.c

@@ -658,6 +658,12 @@ int coap_resource_parse_observe(struct coap_resource *resource, const struct coa
 		ret = coap_service_remove_observer(service, resource, addr, token, tkl);
 		if (ret < 0) {
 			LOG_WRN("Failed to remove observer (%d)", ret);
+			goto unlock;
+		}
+
+		if (ret == 0) {
+			/* Observer not found */
+			ret = -ENOENT;
 		}
 	}