function getMessageProperty(msg,expr) {
    if (expr.indexOf('msg.')===0) {
        expr = expr.substring(4);
    }
    return getObjectProperty(msg,expr);
}
Gets a property of a message object.

Unlike {@link @node-red/util-util.getObjectProperty}, this function will strip `msg.` from the
front of the property expression if present.

@param  {Object} msg - the message object
@param  {String} expr - the property expression
@return {any} the message property, or undefined if it does not exist
@throws Will throw an error if the *parent* of the property does not exist
@memberof @node-red/util_util