feat(xo-cli rest): explicit error if not registered

Fixes https://xcp-ng.org/forum/post/68698

CHANGELOG.unreleased.md

@@ -35,6 +35,7 @@
 <!--packages-start-->
 
 - vhd-lib patch
+- xo-cli minor
 - xo-server minor
 - xo-web minor
 

packages/xo-cli/rest.mjs

@@ -134,6 +134,12 @@ export async function rest(args) {
 
   const { allowUnauthorized, server, token } = await config.load()
 
+  if (server === undefined) {
+    const errorMessage =
+      'Please use `xo-cli --register` to associate with an XO instance first.\n\nSee `xo-cli --help` for more info.'
+    throw errorMessage
+  }
+
   const baseUrl = server
   const baseOpts = {
     headers: {