immutable file download: make the ciphertext hash tree mandatory

immutable file download: make the ciphertext hash tree mandatory
This fixes #491 (URIs do not refer to unique files in Allmydata Tahoe).
Fortunately all of the versions of Tahoe currently in use are already producing
this ciphertext hash tree when uploading, so there is no
backwards-compatibility problem with having the downloader require it to be
present.