3835: PySDK raises NotFoundError when all Keep services report such.

3835: PySDK raises NotFoundError when all Keep services report such.

Previously, we raised this error when >= 75% of services reported
such, as the most reasonable available cutoff to make the
distinction.  Now that Keep exceptions include detailed information
about the error from each service, it seems useful to make this
threshold stricter, and only raise NotFoundError when we're sure
that's the problem.  See further discussion from
<https://arvados.org/issues/3835#note-11>.