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 just 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 just send the exception object instead.
Add PDFJS.abortGetDocument function to enable cancelling of a call to PDFJS.getDocument