Improve how errors are habndled when connecting to a server

[acelaya]

• Display an error when selected server is not found.
• Display an error when could not connect to selected server.
• Handle connecting to a server in just one place (the MenuLayout). From anywhere else, it should be possible to just navigate to the proper URL with the proper server ID.
• In the first two cases, display the list of configured servers to allow the user select another one easily.