Add failure recovery for Suite/Unit/Task exection

[rodneyrehm]

It should be configurable how dalek should proceed when a task (action, assertion) fails.

• abort instantly (current behavior)
• continue with the next task
• continue with the next unit (skipping following tasks in same unit)
• continue with the next suite (skipping following tasks in same unit and following units in same suite)

beforeSuite, beforeUnit, afterUnit, afterSuite likely have a different requirement for failover.

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dalekjs/dalek#13 (dalek 0.0.x was continueOnFailure by default)