Rescue Kubeclient::HttpError when generating prometheus_client

What does this MR do?

Currently users are unable to see environment pages if the clusters are improperly configured. This MR fixes the 500 error.

• Sentry: https://sentry.gitlap.com/gitlab/gitlabcom/issues/192832/
• ZD: https://gitlab.zendesk.com/agent/tickets/95412
• Issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/46406
• Slack: https://gitlab.slack.com/archives/C0SFP840G/p1526132883000135

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Related: https://gitlab.com/gitlab-org/gitlab-ce/issues/46406