Correctly handle WorkerErrorEvent
(related to #17)
Don't use OS.File API in Firefox < 20 + use "tmpPath" in Firefox < 25
Buggy on startup in Firefox 18 and 19:
Error: reference to undefined property options.bytes
Source: resource://gre/modules/osfile/osfile_shared_front.jsm
Doesn't work without tmpPath option in Firefox 18 - 24:
TypeError: Expected option tmpPath
Source: resource://gre/modules/osfile/osfile_shared_front.jsm
(#17)