virt-viewer-file-transfer-dialog: improve error message
    
    This patch improves the error shown to the user when a file transfer
    fails.
    
    The previous behavior was to create a simple message dialog box, with
    the error description and the full list of the files that failed to be
    transferred. When the list of files was long, the dialog box would
    grow bigger than the screen.
    
    Now, the file list is inserted inside a scrollable widget, whose
    height is limited to 170px.
    
    NB: these two calls would be more adapted, but they require GTK >=
    3.22:
    
    > gtk_scrolled_window_set_max_content_height(GTK_SCROLLED_WINDOW(scrolled_window), 170);
    > gtk_scrolled_window_set_propagate_natural_height(GTK_SCROLLED_WINDOW(scrolled_window), TRUE);
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1496356
    
    Signed-off-by: Kevin Pouget <kpouget@redhat.com>
    Acked-by: Victor Toso <victortoso@redhat.com>