CI: Catch missing pipeline information

Very occasionally, pipeline data for a build is unavailable (e.g.
due to API failure or a project rename).

When querying the Gitlab API, always check the status code and
return the build status "Unknown" if the HTTP code is not "OK".
This makes the script more robust to invalid data.

Update the template to render the "Unknown" status.

Signed-off-by: Peter Hoyes <peter.hoyes@arm.com>