mr widget: exercise greater paranoia about gitlab responses

Attempt to avoid any breakage when GitLab is unavailable or responds in an unexpected way:

• Guard against foreach over anything that is not an array in getMergeRequestsForTask()
• Wrap getMergeRequestsForTask() calls in try-catch
• Wrap request via HTTPSFuture in try-catch
• Check response for a 200 on API request
• Guard against a non-array result for json_decode() of response

Also tidies cache logic a bit and removes some debugging boilerplate.

This is a whack-a-mole response rather than a well-reasoned rewrite from scratch, so something could still explode.

Test notes:

• Set private $task_override = "324164"; to check for a displayed widget for a task with known linked merge requests.
• Nothing at all should be displayed if you hit a URL which 404s, or a nonexistent host.

Bug: T333347 Bug: T335504