Handle IO exceptions from tahoe-great-black-swamp

Try as hard as we can to make ``download`` return its failures in a Left
instead.

Also factor most of the tahoe-great-black-swamp parts out of `makeServer`in to
a separate helper function.  I thought this function might be where the
exception handling goes but it can't really yet because StorageServer requires
IO exceptions as part of its interface.  Should likely fix that sometime.