Add phase name to error messages in Syntax.ml

[jjcnn]

As part of the multistaging issue, generation of error messges has been moved from TypeChecker.ml to Syntax.ml. However, the generated error messages reports type errors regardless of which phase causes the error.

The error message functions should somehow extract the phase name, either simply by accepting the phase name as a parameter, or through the enclosing functor somehow.

[anton-trunov]

Is this still an issue?

[jjcnn]

I believe so. I don't think anyone has fixed it.