The Promise you get from submitBatch Throws an error if its not succesful. You have to pass it a second function where you handle the error.

oModel.submitBatch("cmpchange").then(
    function (oEvent) {
        let messages = this.oContext.getBoundContext().getMessages();
        if (messages.length > 0) {
            let oJSONModel = new JSONModel();
            oJSONModel.setData(messages);
            this.oMessageView.setModel(oJSONModel);
            this.oMessageDialog.open();
        }
    },
    function (oError) {
        // here you get the error in oError
    }
)
.bind(this);