Add an all-or-nothing mode

[machawk1]

There are times when, if one archive fails to respond to memgator, I don't want a response from a client querying for results from memgator. This would be different from an archive responding with a 404 or zero mementos. From the client's perspective, I want either the mementos aggregated from all archives configured or some other indicator that this aggregation was not accomplished, either through some addition HTTP response header, HTTP response code, or some indicator in the body.