Make doctor warnings not count as errors

What does this merge request do and why?

This makes gdk doctor warnings not count as errors.

We currently track non-zero exit codes (including false) from commands as errors in telemetry. Since gdk doctor nominally fails if there are warnings, we count warnings as errors, even though warnings are an expected consequence of regular gdk doctor use.

Closes #2702 (closed)

How to set up and validate locally

1. Run gdk doctor
2. If there are warnings, see that the exit code is still 1

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 MR references an issue describing the change.
• 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.