Another try at propagating errors from bang-methods.

This has one particularly nasty side effect that I know of.  If the
application database doesn't exist, CouchDB returns status code 404,
which is appropriate *for CouchDB*.  However, for the *application*,
that should really be a 5xx class error.

On the other hand, I want to be able to know about CouchDB failures as
transparently as possible.

I'm still tweaking this error-reporting mechanism, so expect changes in
the near future.

(One possibility is that bang-methods in Analysand just have to go away.
They're awfully convenient, though, so I do not want to give them up.)