[Maintenance] Use checked locks for call count and thrown errors

[graycampbell]

What area needs maintenance?

As pointed out here, callCount does not need to use an unchecked lock, it can use a checked one. The same thing should apply to thrownErrors.

Why is this improvement needed?

Using checked locks is stricter and more correct since callCount and thrownErrors use Sendable types.

Proposed solution

Use checked locks for callCount and thrownErrors.

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