Make GitMaintenance diagnostic more failure resilient

Overview

The following discussion from !3848 (merged) should be addressed:

• @toon started a discussion: (+1 comment)

  I think we need display_error: false as well.

  It's rather confusing to see:

  $ gdk doctor
  ...............❌️ ERROR: 'git config --global --get-all maintenance.repo' failed.
  .... 

In !3848 (merged), we added display_error: false but we should check success? of the Shellout and improve error handling, so we don’t show all repos if the git command fails.

Impacted categories

The following categories relate to this issue:

• 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.