Fix an error reporting some failures in the elasticsearch indexer

What does this MR do?

Sometimes Gitlab::Popen.popen returns nil for status. Presumably this represents an error forking, but we don't handle it correctly and so we get a NoMethodError rather than the actual problem.

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

Why was this MR needed?

Bugfix

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #3516 (closed)