Ensure that `getMainThreadWorkerMessageHandler` won't accidentally break `getDocument` (PR 10139 follow-up)
Ensure that `getMainThreadWorkerMessageHandler` won't accidentally break `getDocument` (PR 10139 follow-up)

*This should have been part of PR 10139.*

In the event that a user has attempted to manually load the worker file on the main-thread, but somehow failed to do that correctly, there's a possibility that `getMainThreadWorkerMessageHandler` could throw. Considering how/where that helper function is being called, an error could still prevent `PDFDocumentLoadingTask` from completing (regardless if it's being resolved/rejected).

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