Improve testability and errors of Blob #210
Description
I reviewed the test code for Blob as part of #203 and found it wanting.
Currently we can only really test serialisation/deserialisation as part of a server, unless I put an unreasonable amount of effort into mocking. This PR makes the blob data manager a module-level global: a near-future PR would be able to eliminate this nasty bit of context-dependent code, if we implement a nicely-serialisable UrlFor object. That's probably best left for v0.0.13 but it would be nice to do.
Once that's done, we should revisit the test code for Blob and also review the exceptions it throws, as they could be more helpful.