CORE-229:save a workflow message when aborting due to cost limit

[davidangb]

Ticket: https://broadworkbench.atlassian.net/browse/CORE-229

When a workflow is aborted due to running over its cost limit, save a "Cost limit reached. Workflow was aborted to prevent cost overrun." message to that workflow. That message will be displayed in the Terra UI to indicate what happened with that workflow (this may require additional PRs)

As a bonus, I've addressed an outstanding TODO in the code where we were issuing db update … statements on every monitor pass for each workflow (if a cost cap was set), even if the workflow's cost and status hadn't changed.

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