Catch missing Ruby version and explain how to recover

What does this merge request do and why?

1. Adds debug output to troubleshoot issues with GDK, like #1697 (comment 1433398237)
2. Catches gdk failure when asdf complains that it can not find Ruby, which happens on Gitpod restart
3. Shows actionable error message (gitlab#387476) with recovery instruction

How to set up and validate locally

Open old Gitpod workspace created with outdated docker image.

1. Run

$ GDK_DEBUG=1 ./bin/gdk

2. See

GDK(DEBUG) gdk_root_path: /workspace/gitlab-development-kit
No preset version installed for command ruby
...

3. See that asdf instructions are present

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.