Improve error messages for workspaces setup script
The following discussion from !169739 should be addressed:
- [ ] @vtak started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/169739#note_2166236129): (+8 comments)
> **Author notes**
>
> This logic ensures that (almost) any subsequent upgrades after the first-time installation are very straight-forward where it gets the values from the existing chart if they are not set and are required.
>
> Since the latest release had some change in the structure of the secret, you'll have to run this script as
>
> ```shell
> CLIENT_ID="UPDATE_ME" CLIENT_SECRET="UPDATE_ME" ./scripts/remote_development/workspaces_kubernetes_setup.sh
> ```
>
> After this, you should be able to run it directly without any environment variables and the script will try to gather the required information.
>
> ```shell
> ./scripts/remote_development/workspaces_kubernetes_setup.sh
> ```
issue