Adding Error alert if Observability UI is not loaded within certain time

What does this MR do and why?

Issue Link - gitlab-org/opstrace/opstrace-ui#159 (closed)

• Added error alert if Observability UI is failed to load within 20s.
• gitlab-org group was hardcoded for skeleton changes in previous MR, removed the hardcoded value.

Screenshots or screen recordings

How to set up and validate locally

1. This does not require running Observability UI locally. as we are testing error alert which should come when observability UI fails to load. If you want to run Observability UI locally follow detailed steps for running it with gdk are here.
2. Go to a group page, e.g. http://localhost:3000/gitlab-org
3. Access the Observability tab from the left navbar. There will be three submenus Dashboards, Explore and Manage.
4. Visit page http://localhost:3000/groups/gitlab-org/-/observability/dashboards
5. After 20s error alert will display after showing skeleton.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.