Add error message when a need to be loaded ressource is not known
Add error message when a need to be loaded ressource is not known

During XMI deserialization, many errors can occur, related to the XMI
format or to the EMF part. During deserialization, errors can be:

* a metamodel is not found
* an object cannot be resolved
* a ressource is missing.

Regarding the last point, the future version will have to deal with
proxies (enabling "reflexive" relationshipt between resources). At the
moment, this version raises a new TypeError with the problematic URI.

pyecore/pyecore,aranega/pyecore