Add message to error that happens when executing an action without a connector

[mdutoo]

When there is no connector impl specified, ex. OCCI infrastructure when running MART server right after checking it out, a java.lang.UnsupportedOperationException is raised but without any error message.

Creating this exception with an error message ("No connector / impl found for action Y in extension X, you probably add it in your (runtime / MART server) dependencies") would help the user to know where it comes from: not from MART, not from connector impl itself, and from which extension precisely.

Encountered in cgourdin/MartServer#50

[cgourdin]

It is to do in generic connector on methods that throws UnsupportedOperationException.

[philippemerle]

UnsupportedOperationExceptions are generated by the Ecore to Java generator from EMF.
As this generator is used by everyone doing EMF we could not change that.
In connectors we overload generated operations and then implements the right behavior.