Disables filepicker on old Chrome versions

Displays an error dialog if the user tries to mount a local directory, when the version of Chrome does not support this operation.
It is defined, and even these functions are defined (though they will not work).

The general framework is that colab.app.appMode is a boolean variable that is True when in app mode and False when in web (Classic) mode.  No other functions should get called when in app mode.  In this case, the "Open Directory" menu item is deleted when not in app mode, so this code is never reached.