Wrap calls to add_shortcut_folder() in try { ... } block.

Gtk::FileChooserButton::add_shortcut_folder() throws a Glib::Error
exception if the folder being added already has a shortcut, causing the
'New session' dialogue to appear empty when choosing 'New...' from the
Session menu. This happens if Ardour's default session folder is
set to a location that has a standard shortcut in the Gtk file chooser,
e.g. the user's home directory, desktop, &c.

Catch the exception in ArdourStartup::setup_new_session_page(), so that the
'New session' dialogue appears as expected, and likewise in
ARDOUR_UI::open_session() to avoid a similar problem there.

Fixes #5591.