Ensure transport is destroyed if the document fails to load

Currently if a document fails to load, the transport created is never cleaned up. This abandons a webworker which no longer needs to exist. The following document can be used to recreate this on the pdf.js demo viewer (and is re-producable on master)

https://www.dropbox.com/s/1x5yoyp449ab5k7/broken.pdf?dl=0

Video of issue: https://www.youtube.com/watch?v=PC81V7_4q0M