Need to retry on 404 response from Notary

[von]

From the Perspectives-Server README:

The server implements "on-demand probing", so if you query for a service-id that is not
in the database, the notary will automatically kick-off a probe for that service. The
notary will respond with an HTTP 404, and the client should requery to get the results.
The Perspectives Firefox client will requery appropriately.

Currently the pyPerspectives code doesn't handle this.

[von]

Copied from von/PerProxy#13