CKAN doesn't fail gracefully when future registry versions are detected

[pjf]

We can pick up when a future CKAN version has been detected (ie, the user has backgraded their client to an incompatible version), but we don't. As such, we sstill get the errors we should be able to avoid.

[pjf]

This should be a simple matter of testing to see if registry_version is from the future in Registry.cs.