Previously, requests would fail silently by returning an empty struct
in the response.
With this change, responses will properly report fail with the internal
error.
Unless you're familiar with the way the swift source code is organized,
it's not clear what "SourceDocInfo" means, or how it is different from
DocSupport, etc. Move the tests into directories that are named based
on their request (note: we already had one test under CursorInfo, which
just made things even more confusing).
