Accessing a model from HBase throws an obscure NPE if model's EPackages are not registered

[gdaniel]

The resolveInstanceOf() method embedded in the HBase store throws a NPE if one of the EPackages of the underlying model are not registered. We should throw a better exception that explains why the instance cannot be resolved.

Blueprints and MapDB backends may contain the same issue.

[yvrng]

Since #69 : ClassDescriptor#get() throws a NullPointerException with the reason "Unable to find EPackage for URI: {uri}" when the concerned EPackage is not registered.