Handle 404's and other network failure's better.

[tcoulter]

When we make a request, and Geoserver goes down, or there's a network failure, or a timeout, or something else that's odd...; we should handle those, and provide nice, context sensitive error messages like "Could not add the note because XXX".

Right now, stuff just breaks. If this breaks at the right time -- like, if a save fails -- and there's no visible failure, people can lose a lot of work and a lot of time.