Fix the exception propagation when rejecting workerReadyCapability

Currently when an exception is thrown, we try to reject `workerReadyCapability` with multiple arguments in src/core/api.js. This obviously doesn't work, hence this patch changes that to instead reject with the exception object as is.
In src/core/worker.js the exception is currently (unncessarily) wrapped in an object, so this patch also simplifies that to directly send the exception object instead.
Add PDFJS.abortGetDocument function to enable canceling calls to PDFJS.getDocument