Don't raise error if machines directory is missing on machines listing

Tomasz Maczukin requested to merge fix/machine-listing into master

What does this MR do?

Fixes Runner behavior when using autoscaling on a fresh host (where docker-machine wasn't executed yet neither manually nor via the Runner).

Why was this MR needed?

Problem was introduced with changes added in !418 (merged).

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

Does this MR meet the acceptance criteria?

  • Documentation created/updated
  • Tests
    • Added for this feature/bug
    • All builds are passing
  • Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?

Fixes #2008 (closed)

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