handle docker state corner case triggered by workers going down

[mlin]

Stress-testing revealed that a task whose assigned worker goes down can end up in one of three distinct modes:

1. state = running, exit code = -1
2. state = shutdown, orphaned, or remove
3. "desired state" = shutdown, orphaned, or remove (with non-terminal state)

The first case is common, while the last two are rare & appear only in stress-testing with artificially-induced worker shutdown 😢

[coveralls]

Pull Request Test Coverage Report for Build 1770

• 17 of 17 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 96.722%

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Totals
Change from base Build 1756:	0.04%
Covered Lines:	5489
Relevant Lines:	5675

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💛 - Coveralls