Safer node_status master selection and interaction
Safer node_status master selection and interaction

Summary:
This aims to fix two problems:
  1. Contacting a master for status can cause a 500 if there are network errors.
  2. In some cases, we pull the master from unfinished (and potentially quite old) jobsteps due
     to sorting.
By turning any fetching exception into a warning rather than a 500 and only picking master
from the most recently _finished_ JobStep (much more likely to be recent), problem avoided.

Test Plan: None. halp.

Reviewers: mkedia

Reviewed By: mkedia

Subscribers: changesbot, mkedia

Differential Revision: https://tails.corp.dropbox.com/D129495

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