app: Don't leave a window opened in case of connection error
    
    Since commit ed9b3f3 the main window is not hidden when disconnecting.
    But it also is not hidden when a connection error occurs, leaving a
    black display with a not so accurate message to the users in case they
    try to connect to a non-valid address from the remote-viewer connection
    window and in this case the main window (display #1) shuldn't be shown.
    
    The impetus for this chance is the following:
    - user runs remote-viewer without any argument
    - the remote-viewer connection window shows up
    - user attempts to connect to a non-valid address
    - a dialog pops up indicating a failure connecting to the graphic server
    - the main window shows up saying "Connecting to the graphic server"
    - user clicks 'Ok'
    - the main window stays there with the same message
    
    As a user, I expect the program to not show the main window in
    connecting failure cases. This patch accomplishes that.