使用VBA退出Excel会导致运行时错误424

I have been writing a VBA macro that opens a HTML document within Excel (in order to perform various calculations on it). Excel will search for the HTML document within the current folder. If it can't find it there it will produce a file open box where the user can browse to the location of the HTML document manually. All good so far. However should the user select Cancel (rather than selecting a file), I want Excel to display a message and quit. I have been writing a VBA macro that opens a HT