In the case where the document was destroyed, we were rejecting the `Promise` in `JpegDecode` with a string instead of an `Error`. The patch also brings the wording more inline with other such rejections.
Use the `isInt` utility function when validating the `pageNumber` parameter in `WorkerTransport_getPage`, to make it more obvious what's actually happening. There's also a couple more unit-tests added, to ensure that we always fail in the expected way.
Finally, we can simplify the rejection handling in `WorkerTransport_getPageIndexByRef` somewhat. (Note that the only reason for using `catch` here is that since the promise is rejected on the worker side, the `reason` becomes a string instead of an `Error` which is why we "re-reject" on the display side.)
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