api/server/httputils: service unavailable for disable swarm

When swarm-mode is disabled, we need to return an error indicating this.
406 was chosen for the "Not Acceptable" verbiage, but this code has
specific semantics in relation to the `Accept` header, which aren't
applicable here.

We now use a 503 for this case. While it is not a perfect match, it does
make it clear that the particular "service" (read: API endpoint) is not
available. The body of the message provides the user with enough
information to take action on it by enabling swarm-mode and ensuring the
service is available.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

api/server/httputils/errors.go

@@ -62,7 +62,7 @@ func GetHTTPErrorStatusCode(err error) int {
 			{"wrong login/password", http.StatusUnauthorized},
 			{"unauthorized", http.StatusUnauthorized},
 			{"hasn't been activated", http.StatusForbidden},
-			{"this node", http.StatusNotAcceptable},
+			{"this node", http.StatusServiceUnavailable},
 		} {
 			if strings.Contains(errStr, status.keyword) {
 				statusCode = status.code