And try more than one if necessary / possible

Also add SDMF instances for them and tweak the tests to work with the new
tahoe-ssk encode interface.

StorageServer hides all of the networking details in the real implementation.
If tahoe-great-black-swamp works then the existing success case should
exercise all of the same logic in gbs-downloader whether we're actually using
it or not...

It might be nice to move all of the StorageServer implementations into
gbs-downloader and then test that they really do behave the same as each
other. Right now they're mostly in tahoe-chk though so that's annoying.

a bit, anyway

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.

The requests time out for some reason but at least we're trying something.

Also sketch out a demo program

quite possibly both of these interfaces are wrong
may want to ponder them for a moment before proceeding

It fails because we didn't finish implementing it yet, and also because there
might be some more bugs.

Co-Authored-By:  <shae@private.storage>

somehow I forgot to write the test for this bit first
too exciting to actually be implementing a downloader I suppose