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.