Global default statement: Improve code & error message - Simplify error handling. - Improve error message: add message ID and relevant quotes and try to standardize a bit. - Add proper doc for in the message registry. Note: Instead of testing the value of `expr.op`, we test if the argument `elem` of `ExprResolveLhs` is set: this allows us to catch also the error with `x.y[z]` rather than just `x.y` as previously.