Fix GitLab check: Problem with Elastic Search

What does this MR do?

• Adds exception handling the the SimpleExecutor (will be backported to CE)
• Fix ElasticSearch #check? logic
• Adds #show_error linking to documentation

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

Are the versions we are supporting correct? (5.1 - 5.3)?

Why was this MR needed?

There was a regression with the check migration from old check.rake to the SystemCheck: see #2757 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #2757 (closed)

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12487