Better exception management from connector errors on occiCreate (occi crud methods)

[cgourdin]

Actually connectors does not throw exceptions (not runtime exception) on CRUD methods (occiCreate(), occiDelete etc.). Find a way to get exception if there is one thrown in these methods.

[philippemerle]

A possible solution is to add 'throw Exception' to all occi* methods.
However Exception is too generic, so we could prefer to define a specific OcciException andd add it to all occi* methods.