Return partial data on error?

[carolynvs]

When retrieving a list of entities fails, can we still return a partial list of entities and an error? Right now, this applies to ReadAll, where we may hit an error reading a single value and it would probably be good to continue returning what we could read and let the tool decide if they want to display partial results and deal with the error.

For example, one of the items may have been marshaled using an older data format but something like that causing the entire list of items to be unretrievable would make the situation worse.

After #212, there are even more functions that return lists of items and this concern/strategy applies. Specifically if we are listing installations, and one has a problem, it would be a real problem if that prevented us from listing ALL installations.

So let's come up with a pattern for how to indicating that a partial set of data was returned along with the error. This most likely has strong overlap with any solution for adding concurrency to our persistence layer (#214).