More Robustly Handle Errors Checking Sizes

We experienced a problem connecting to a hosted queuing service, which made the /okcomputer/all check fail entirely. We'd rather the SizeThresholdCheck handle exceptions more gracefully, reporting within its check that something went wrong and providing relevant details.

QA Plan
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1. Use this branch in an app with a size threshold check. Add `raise "Something terrible happened!" in your size check before returning an Integer.
2. Confirm that you receive a failed check when viewing /okcomputer/all.txt rather than the entire set of checks blowing up.