Add support for emiting warning / errors from cecilifier

[adrianoc]

As of today whenever we want to call user's attention to some aspect of the generated code we have two alternatives

1. emit a comment in the generated code: far from ideal, users may simply overlook it.
2. throw an exception: Ugly, prevents user from seeing any generated code; may mislead users to think they found bugs

We intend to add a mechanism to allow Cecilifier to emit warnings/errors in a way that will be harder to overlook them.

API

internal interface IVisitorContext
{
   // ... 
     void EmitWarning(string message);
     void EmitError(string message);
}

Bebavior

Invoking these members will:

1. Show the message in the frontend (as a notification, similar to the notifications used to report compilation errors)
2. Add a #warning message / #error message directive in the generated code so if user overlooks 1 they have another chance notice the warning in the code and if they don't and try to compile they will get a compilation warning/error.

Known use cases

The main driver for this feature is to report unsupported C# features but most likely there are other use cases.